URF Custom games?

Why can't we make (AR)URF custom games?

And why is URF only on Summoner's Rift?

Like, are we not allowed to have fun by Riot or something?

I'd like to be able to pick a champ I like and spam it's abilities against my friends and enjoy the game, which ARURF in its current shape kinda limits. You get forced to pick a random champ in a temporary gamemode and you can't even try out all the possibilities because you're matched against others and a competitive environment is created...