Why..

So new players don't go into PvP assuming it's okay to AFK.

Because you are playing with real people... It is sad I know but you are getting matched with people that you do not really know.... But in custom game the leader can kick you out, they can joke around play mini games all that stuff... It's sad like for example they should make it that @ lvl 20 or something you shouldn't get leaver buster since people already know the game.

I get my friends account 20min low priority quence in bot match LOL, it was hilarious, at that moment I almost die of laughter, laugh at how stupid the system is. XD