For 4k video, H.265/VP9 have better quality at the same file size, or equivalent quality at a smaller file size. Bandwidth usage matters for keeping people on the website and viewing Google's ads, so Google wants to ensure a smooth streaming experience. And since YT compresses so heavily, uploading in 4k increase the quality at lower resolutions (it seems weird but a 4k video uploaded to YT and streamed at 1080p will look better than a 1080p video uploaded to YT and streamed at 1080p).

And as xnyan said, high motion video really destroys video quality on YT, this video really shows how bad it can get: https://youtu.be/r6Rp-uo6HmI

It's a huge topic and I'm not a professional, but tldr Google's implementation of h264 compression is not suitable for high motion video and the changes they would need to make in order to make it suitable are not worth it in view of the trade offs of size and other factors.

Why can't a $1K general purpose computer efficiently decode a royalty-free codec (hint - it may have something to do with the fact that apple has a major stake in a competing for-pay closed codec). I really do like apple, but I see apple as microsoft 2.0 in this.