TRoS combat is about not being hit
Let's back up a few steps to deal with some mistaken assumptions that are leading you astray. Then we'll see why aiming for the head is a great idea, except when it's not.
Armour makes you easier to be hit
Paradoxically, armour can be the cause of being hit. Yes, it can decrease the severity when hit, but you'd rather not get hit at all, right?
In TRoS (and real life), wearing armour slows you down and fatigues you faster.
A good pot helm is AV 5. Good over-all protection from a chain suit is less, only AV 4. But the helm will give you a −1 CP penalty, and the chain suit will give you −2 CP. Together, that's −3 CP — three fewer dice that this opponent has for defensive maneuvers to prevent you from hitting them.
Fatigue is a lethal killer. If fatigue rules are used, you are on a shorter clock when wearing armour than wearing little or no armour. In combination with initial CP penalties, armour starts looking a lot less of an obvious choice.
For example, someone with a decent Sword and Shield Combat Pool of 10, with an average Endurance (EN) of 4, loses 1 CP every four rounds (8 exchanges) when wearing that kit above (on top of the −3 CP to begin with just for wearing it). Against an equally-matched CP 10, EN 4 opponent, that means that after taking or receiving a mere 8 swings, they'll have a CP of only 6. On top of that, they may already be on the defensive and unable to safely attack, since they started with a lower pool (CP 7 vs. CP 10) and have had to fight more conservatively.
Meanwhile, their equally matched EN 4, CP 10 opponent wearing only a pot helm is enjoying a Combat Pool with 9 dice in it for a further 4 rounds before their first 1 CP lost to fatigue. Who wants to bet that the armoured guy will get hit first?
Getting hit at all probably means you've already lost
TRoS combat is not about insta-kills, it's about degrading your opponent's ability to continue fighting faster than they can degrade yours. Fatigue, shock, pain, and blood loss are your offensive tools, not insta-kills. Insta-kills are the gravy, not the meat and potatoes.
Yes, a shot to the head is devastating — but you're probably just as dead if you're gently sliced across the arm and you've gone into shock (4 dice), are bleeding (2 EN rolls per Round), and fighting through agony (5−WP pain dice), because you likely can't defend yourself effectively enough next Exchange (obviously they were good enough to hit you when you had your full CP!) and you're about to be finished off. Or captured! Capture happens, since so many non-lethal injuries can so severely make you vulnerable to “surrender or die!” ultimatums.
Continuing our example, the less-armoured guy above suddenly gets tagged with a lucky slice, suffering a Moderate wound to their arm. Suddenly that 9 CP pool is down to 5 CP (and possibly less or even empty, if they have already attacked or defended and used some CP already), and next Round their CP refreshes to 9 CP − Pain. That means Armoured Guy can go fully defensive and let Less-Armoured Guy bleed out (2 EN checks to decrease Health each Round), or can press the advantage and try force LA Guy to put all CP into defense to try to stay alive.
The contrary situation is even worse: that CP 7 Amoured Guy is already more likely to be hit by the CP 9 Less-Armoured Guy. If LA Guy tags AG first for even a light hit through that armour (more likely too, due to having a few more dice), suddenly it's CP 9 against maybe CP 3 or worse, and AG is as good as already dead (or captured).
Armour is only insurance
Armour can turn a bad, bad hit into a “I'm not dead yet” hit. Even then, you might still be dead and just not know it yet, because of the aforementioned dynamics around being injured at all.
Further, there is only so much that armour can do for you. It's an insurance policy against “hits that would have been a little bad”. (And ironically, putting on armour in the morning might mean you've already killed yourself, what with it being bad for your Combat Pool, as we've seen.)
A pot helm of AV 5 can turn a sword strike from “Skull is shattered. Character is unconscious and may not recover” into nothing, but the difference between no damage and “Real, real messy. Instant death” is only a range of 6 successes, and armour AVs are in the 2–6 range, so your margin of error is really narrow and AV only shifts the result across that narrow window. In other words, armour is only likely to help, because a sufficiently high number of successes is still going to maim you enough, even with armour in the way, to likely mean you overall lose the fight.
And that's talking swords. If they're swinging a club or mace at your head, even one success is likely Game Over since even 1 success past the AV has devastating effects on your CP.
This is making it sound like swinging for the head is really great though, isn't it? But here's the real takeaway: armour for any location is only just kind of effective, and sometimes it actually makes things worse for you. So likely your opponent isn't wearing armour of equal AV everywhere — likely, their head is the highest AV location you could choose, and you'd be more likely to hit an arm or calf instead.
Remember how combat is all about hitting at all, rather than insta-kills? Those bare arms are going to be tempting targets, because even one or two light slices (Damage 1) can decrease their fighting ability enough to be the turning point that wins you the fight.
So which is better? Armoured head (for lucky devastation) or unarmoured arms (for attrition)? That's a tactical choice now.
TRoS is about tactics choices, not bigger numbers
A skilled player with good tactics and who can accurate evaluate the strengths and vulnerabilities of their opponent (where are they armoured? how is my fighting style, weapon, and known maneuvers against theirs? do they favour risky offense or conservative defense, and can I exploit that? how should I divide my CP across Exchanges and defense/offense? can I taunt them into suboptimal decisions? should I dodge or block or feint? how dangerous is their weapon and their tactics against my strengths and vulnerabilities? should I alpha strike?) will often beat an opponent with strictly superior numbers on the character sheet.
Your opponent won't be fully armoured unless it's good tactics for them. If it's good tactics to penalise their CP that much you should be worried and looking out for how they offset that disadvantage, instead of whacking away at their head. Yes, in that case their head is probably the best target, but while you're doing that, they're might be anticipating it and planning something that rock-paper-scissors exploits your straightforward attack.
If it's not good tactics to be fully armoured, then the head is still a great choice, but it's no longer the obvious choice. A slice at Zone IV might be better if they have a pot helm, because it has a chance of passing under their helm and cutting their neck, or hitting their upper chest (where there are some solid damage outcomes). If their legs are vulnerable, you can severely hamper their fighting ability with margins of success that, if they hit a helmeted head, would be no damage at all.
In general, it's better to try to hit someone at all, anywhere for 1 or 2 damage than to try to get past armour to get the juicy damage effects of heads or torsos.
Don't get hit.
In general, wear the minimum amount of armour you can get away with, and expect your opponents to do the same. But mostly don't get hit in the first place.
Try to tag the other person, even if it's a little cut, so that they fail to not get hit.
Don't expect your average opponent to be fully armoured, and fear the confidence (and wealth) of the one that is.
Put armour you can afford (in price and CP penalties) where you can't afford to get tagged, but only because it might save you from a nasty hit someday or will encourage your opponents to stay away from that hit location.
Most importantly, don't get in fights where your Spiritual Attributes aren't giving you major CP boosts. TRoS isn't about skirmishing with every rude person you meet, because the odds over time are stacked against you and that's how you die. TRoS is about fighting and dying only for what you believe in, so get into fights where your Spiritual Attributes both demand that you do, and will tip the odds in your favour so that you can do things you couldn't otherwise and are more likely to win — and even if you lose (and you might still), you will have died a glorious death fighting for True Love or whatever your SAs are.