Their playerbase is the highest it's ever been at 100mil+
It's just that Plat/Diamond players rarely play anything aside from ranked, so their matchmaking has to put them in games with lower tier players. If they have 20+ minute queue times for Ranked, where most people in that rating are playing, imagine what their queue times would be in normals were they locked to playing in their rating.
It's also true that normal and ranked MMR's aren't linked at all, which, while I feel like ranked MMR should influence normal MMR to an extent, is also understandable because higher rating players don't play normals very often. If they are, it's either to detilt after a loss streak and just shut their brains off (in which case, playing against low tier players is alright because they won't be playing at their real level) or because they're playing with friends that are low tier (again, fine because they're likely just gonna troll). To think that a Plat/Diamond player is in every one of your normal games playing at a serious level is just plain ignorance about these players' routines.