Why does Shaco have a limit on boxes in URF?

because boxes hard CC you. You cant blink dash or even try to run away if you are feared

Maokai's isn't a box.

He actually does have a limit, if you played him you'll find out. After going over a certain amount of saplings out there, when you toss the next sapling, the earliest sapling you had out there would automatically explode. You can never go over a certain # of saplings even if you setup a colony in a bush.

Also if I may state my opinion, shaco's box is way more op than sapling. They both do dmg, but shaco box will continue to do damage until they're destroyed, and most importantly shaco box is considered hard cc, while sapling is just a slow. There's room to fight back in the latter case, but not the first.