For us, even regular YouTube is substantially louder than any streamer. If we want to watch something on YT than go back to Hulu/Netflix, we always have to adjust the volume. I don’t get it, why, why?

Louder content is more compelling (to a point), so I'd imagine that louder content helps boost watchtime, which is what both Youtube and the video creators are optimizing for. The music industry's "loudness war" seems related.

https://en.wikipedia.org/wiki/Loudness_war

that's odd to me because a lot of the time I've found that regular YouTube content is on the quieter side to other services.