Yes, there is a good reason for feat prerequisites.
Two reasons, in fact:
To clarify by indicating what build a feat is intended for (a feat that boosts
Lay on Hands usually requires that you have
Lay on Hands)
To balance by forcing players to dedicate more resources in order to take a more powerful feat.
I suspect the reason you're surprised to encounter this is that by and large 4e feat prerequisites are for clarity: if there's a feat we want, we probably already qualify for it. (In previous editions of D&D prerequisites were much harsher and planning our characters around qualifying for certain feats was common.) It's surprising to find a feat we want that we can't take.
Mastery feats need to be balanced carefully:
Feats like the Mastery set can be very powerful: they double crit range from 1/20 to 1/10, and for a build that 'fishes' for critical hits that's pretty massive. Consider a burst/blast wizard who attacks an average of 4 targets each round; compared to a character with expanded crit who makes one attack in the same time, the wizard with expanded crit will deal maximum damage every 3 rounds instead of every 10. If he has specialized in making his criticals awesome, this is very powerful.
In order to allow such feats to exist without unbalancing the system 4e has imposed more stringent prerequisites to take them. But...
The requirements really aren't that harsh:
Prerequisite: 21st level, Dex 15, Int 21, Wis 15, wizard
At level 21 two of a character's abilities can have increased by +6, and the other four abilities have increased by +2. This means that for
Wizard Implement Expertise I can start with (before racial modifiers, assuming I take a race with +Int) a 13 Int, Dex, and Wis to qualify at 21 without focusing on either of those secondary stats or having racial bonuses to them. Roughly two out of every three wizards will have focused on at least one of them, and it's important to remember that the 4e stat generating system makes it impossible for me to start with less than a 14 in at least one stat.
A player who wants the feat should have been considering it ahead of time and planned accordingly. Even a wizard who hadn't planned to get the feat is likely to qualify. (If I want it so bad anyway, I've got two +1 stat bumps at 24 and 28 to bring my 11 up to a 13 and still enjoy the feat for the last three levels of the game.)
Sure, change it if you like.
The phrase "feat tax" gets thrown around a lot. It seems to mean that there are specific feats I need to take in order to not be a burden on my party. I don't agree with this idea; it presupposes a specific kind of character optimization which is just one style of play rather than a philosophy of the game design.
Feat slots are limited for a reason: to force choice. There are better and worse choices, though 4e has done a surprisingly good job of limiting downright awful choices. By lifting feat prereqs and granting free feats a party is saying these choices aren't the kind of game they want to play.
And if that's your party's play style, go for it: there's no wrong way to play the game provided everyone's safe and happy. But it's not the way the game was designed, and the game wasn't designed by accident (however much I joke about the adventure writers), so we're less surprised by mechanics like these if we understand the original philosophy behind them.