With wardrobe, plushies, beanbags and pumpkins, when you buy the item there's an option to immediately put it into your collection. With minions, books and food, you have to go to your inventory to use it.

Since books and food are different from collections it's more understandable, but why can't we at least immediately put away minions in the zoo?

I think it's possibly related to the fact that the Minion Zoo isn't like, entirely implemented in some ways? It's a really half-baked feature. Also unlike other collections you can put as many of any minion you want in there, so that might also be part of it, all those other collections only allow one of each item