h264 and h265 are patent-encumbered and therefore very expensive and/or dangerous. Patent trolls would rip Mozilla apart and eat all their money. The only reason H.264 works atm is that Cisco sponsors a plugin for that.

H264 patents are finally starting to expire, all the known patents have already expired in Europe.

As for HEVC, that particular licensing trash fire continues to burn bright. VVC had an opportunity to learn from the situation, and decided what they really wanted was a trash fire that burned even brighter.

So, we might be stuck with H264 for a little bit.