to enforce their LP system, and for no other reason, points must come from somewhere or else everyone scales up the ladder and everyone thinks they're better.
which really doesn't matter, because we could all be playing at the challenger level its just that some players just have to be in bronze because they either dont play as much or happened to lose more than the next person.
which is why the MMR system was better, players were ranked by number which could only be increased by playing more, anyone could do it, you wouldn't be gated by promotional matches, you could just keep playing and if you were better than other players it would show eventually.