In 2009/2010 YouTube advertised Chrome as a better way to view YouTube videos pretty extensively. YouTube also is a moving target that other browsers have to hit (see also GoogleDocs.)
It's similar to how "DOS ain't done until Lotus won't run", only with YouTube and other browsers.
Microsoft abandoned their "not Chrome-based" Edge code base, despite the fact that it was significantly faster than Chrome on almost every other site, because of the way that YouTube does some layers. This "way" has no functional effect, but it does block certain optimizations, optimizations that Edge was using and Chrome still isn't.
Google optimizes YT for Chromium-based renderers. Or rather, Google pessimizes YT for other renderers.
It's similar to how "DOS ain't done until Lotus won't run", only with YouTube and other browsers.