A possible reason riot wants games to be shorter

is matchmaking system.

Given a player plays for the same amount of time, he will be playing more games the shorter games are. Ever since the introduction of role-choosing system in season 6, it had become a lot harder to match 10 players than before, assuming same number of people queuing simultaneously at a given time on average. Riot kept trying to tweak the algorithm to make matchmaking better, but the actual solution to it as we see now, is making games shorter so that people will have to queue more than before to play same amount of time, which would mean more people queuing simultaneously at a given time. And also, rito kept making supports more and more OP to ease their matchmaking algorithm, so that more people queue up for support and matchmaking can actually work.