1. The algorithm absolutely pushes negative content.
2. YouTube audiences want their fave channels to stick to one thing.
3. Negativity has, and always will, be popular.
A YT'er known for playing Minecraft, for example, will be expected to play Minecraft, and videos they do on other topics will perform significantly worse.
Viewers like familiarity.
This has been true in my 10+ years of games media across many mediums.
When it comes to negativity, I try to at least have something worthwhile to say.
I don't blame anyone holding tightly onto their niche.