Practice Tool

Ok so for the training/practice tool modes, where its just you, or just you and a single bot player, with no rewards just training, shouldn't you be able to use and test out literally any champion at all including ones you don't own?

I am sure someone has asked this before or there is an answer to this or why this cannot be done, but I just really want to know.

Its fine that we can't test skins out I guess but why not champions? I mean if we are curious about them, wanna test builds, try them out before purchasing them or permanent unlocking them, and etc., we should be able to do so. We can already get random champion selections in Aram, so why not just let us test out any specific champion we want at will in a purely offline practice match.