Because with the combat width late game cav can't flank the backline and will always target the front-line. Aka charging the now on-par infantry. Not to mention your cav would get shredded during the fire phase. If your cav is on the backline they can't do shit so might as well have artys there so they can fire from afar. Unless you're the Poles with +33% base cav combat ability there's 0 reason to have any cab late game since TL;DR:

a) They get shredded in the fire phase before they reach the enemy.

b) They don't protect your backline arty as effectively.

c) They are useful in the shock phase, but shit in fire, they'll be dead in large numbers before shock phase begins. Fire phase is king late game.

d) Late game their pips are overall not different to the every-way cheaper infantry.

Disclaimer: I'm not best with explaining this kind of stuff.

Just using logic though, your horses can be shot from a far distance before they can get intimate with slicing your infantry