1) it's easier to get the frequencies right than just randomly trying to call or fold the correct percent of all of your bluff catchers
2) sometimes you will misread the situation and your opponent will not understand that he SHOULD be polarized and he will try to value bet some hand that the top of your range would beat but the middle would lose to
3) people do range merge from time to time and try to make a very thin value bet in a spot they would appear to be very polarized and the top of your range may just be able to pick off the range merged thin value bet (if villain believes you will just be calling with your bluff catchers X% and folding the rest and he thinks this spot looks like one where, as you proposed, 22 and AA are ~ then he may just go for a thin value bet with KK or something expecting it to be good against your bluff catches and you win because you used the top of your range to bluff catch
4) players will sometimes turn hands into bluffs. Maybe villain has a hand like 66 and decides to turn it into a bluff and you bluff catch with 22.. OOPS! nothing worse than correctly determining your opponent is bluffing and then making a losing call against him. That's the worst.
5) sometimes the top of our range will be the kind of hand that has good blockers to villains value range and that makes it the perfect bluff catcher but other times it does not and if could even block the bluffs in villains range and in these spots you may well do best to chose the hands that are not the top of your range but, rather, the hands that have the best blocker effects. So it's just a good rule of thumb that you should TEND to use the top of your range to bluff catch
also, here is just a good, simple and practical rule of thumb that I teach people whenever they ask for my help with poker; WHY NOT?
There is rarely a reason to fold hand A and continue with hand B in a given spot where hand A is > than hand B.
It is just a very good habit to get into that we should not fold some hand like AA for an over pair in some spot where we call KK. They may be ALMOST identical but they will almost never be exactly the same because players do strange things sometimes.
So; Simplicity, getting good frequencies, occasionally beating a range merge, occasionally beating a worse made hand that is turned into a bluff, and sometimes people just do dumb things that make no sense and you having a better hand to call with makes it more likely you win in a spot where villain is just randomly clicking buttons. And, most importantly,.. WHY NOT?