Why isn't armor penetration the primary modifier of ADCs and Crit the primary modifier of assassins?

If ADCs counter tanks and assassins are supposed to quickly dispatch squishies while having a hard time with tanks, wouldn't it make more sense?

ADCs build tons of lethality and armor penetration when possible anyway. ADCs wouldn't lose their 'burst' anyway because armor penetration still grants next to true damage against squishies.

Assassins building crit would grant them higher numbers against damage reduction stacking but limit them against actual resistance stacking. AP assassins would still probably stay the same in this scenario though because of the way AP scales.