why cant you use all the champs/skins in practice tool?

Just create a PBE account, literally that easy.

There are over 140 champions in the game.

That is an overwhelming number from the perspective of a new or inexperienced player.

Making them all available to test out could result in sensory overload and generate lack of interest.

Instead, Riot has free-to-play champion rotations every week. 7 days with which to try out one or more of several champions without tossing a mass quantity of options at people.