Probably doesn't help that a bunch of the decent channels were bought by private equity and are now churning out boring, safe and uninteresting content.
https://youtu.be/hJ-rRXWhElI (a yt link, lol).
A brief summary from https://www.dailydot.com/news/youtube-channel-private-equity/
Some channels like Donut Media, Veritasium, and Task and Purpose have been acquired publicly. Others, such as Dude Perfect and Coco Melon, have been acquired more privately, with no public disclosure.
Plenty others. A key giveaway is when a channel diverges their risk. When the front man who is the reason you have watched the channel suddenly has co-hosts and large segments from other channels in their regular content.
But if less people are watching the other big channels because the content quality has slipped, then there is less people spending less time on the platform, so less non-subscribers that might be recommended an LTT video.