[CorpusCalcium]

Sadly, in general this is a case of "we made a Chrome-specific site/app years ago because it was the easiest thing to do, and despite other browsers and some of our employees working hard since then to standardize the Chrome-specific bullshit, it doesn't benefit us enough to bother fixing anything on our end. So we'll just keep maintaining our subpar now-redundant versions for non-Chrome browsers instead."

But luckily this attitude might be poised to change, because the employees at Google who still care about these things have been pushing hard, and it's now threatening to become a PR issue that Google is needlessly serving inferior versions of big apps to other browsers. When other browsers are a UA spoof away from showing that you're artificially screwing with them on your base Search engine, it becomes harder to push those browsers to adopt the web standards you're trying to push to make YouTube "better".