Pokémon Sun and Moon - Swimsuit Edition Pokemon Pals (Lusamine cover)

I assume it is for gameplay reasons, specifically to add a bit of extra tension from possible failure. It could also be to make grinding slightly less tedious, for there wouldn't be a prompt asking the player about using a pokéball each time one takes out the pokémon (which is otherwise deleted upon fainting.)

It might also be due to programming issues, but this is all just speculation.

Yah Oak, why CAN'T they be caught?