Fate is all about creating interesting stories. Progressive stress box values are designed to do exactly that.
The rules say that to absorb Stress, you have to mark off a box that has equal or bigger value than the number of shifts received. If you can't, you have to take a Consequence to reduce those shifts to a more manageable number.
This implies a couple of points:
- If a character is struck by a particularly powerful attack, one that has more shifts than the target's biggest box value, he will have to take a consequence outright.
- If a character is subject to numerous weak attacks, they will fill his Stress track quickly, because a single box can only absorb a single attack, regardless of its shift-value.
- If a character invests into Physique, greater investment provides greater reward, making it an even more notable trait of the character. High Physique not only gives you more total shifts you could (potentially) absorb and more boxes, but also safeguards you from a one-hit-kill.
- Because Stress works the same for every character (save for some imaginative Stunts), you can learn it just once. There are very few fiddly bits that change from encounter to encounter. Once you get past the original learning curve, you have an universal mechanic you can use anywhere.
To compare it with a game using your flat value suggestion:
- Big, coordinated attacks are less important now, as any smartypants who can land
(your_flat_rate)+1 will inflict a consequence. This results in less party cooperation and fewer awesome moments as well. Awesome moments are good.
- Death of a Thousand Cuts is now less appealing, because you won't feel the huge, awesome satisfaction of checking a
 box with a one shift attack.
- Again, making tough characters would be less rewarding. Having
 (8 total, 3 one-hit-consequence treshold) for Physique 4 is not as cool as having
 (10 total, 5 treshold).
- Now, we can probably agree that a flat rate would be just as easy as progressive rate, all in all. Progressive rate requires you to put in more effort learning the mechanics, but the flat rate will require more here-and-now attention and double checking if the value is appropriate. We can probably debate all day whether one is more elegant than the other.
To summarise let me just describe one more situation. In a low-stakes fights, standard mooks would have
 instead of
. Not much difference, your houserule is unlikely to affect the encounter significantly. On the other hand, when the party fights a huge, powerful boss, a dragon perhaps, and you give him
 instead of
 it becomes much less epic, intimidating, rewarding and interesting.
But maybe you still don't like it
All of this said, Fate is also all about what works for you and your group. If you play some Fate games and decide you don't like the Stress tracks, you can ditch them completely, replace them with hitpoints, have only beefed-up consequences or deal damage to character's Fate Point pool. Whatever you like. If you find your group has less fun with progressive stress than without it, remember the Golden Rule.