It's a missed opportunity that we can't just one 1 frame AV1 files as images. Had this been anticipated we wouldn't have to push two standards and two implementations everywhere.
There are already a few formats that work just like JPEG but for modern video formats. Some of them even compress better (FLIF). They just haven't caught on at all. JPEG just has so much momentum.
Those other formats weren't developed by huge corporations who can force it into browsers through pure hegemony. That's probably a factor.
The missed opportunity is that AV1 is arriving everywhere and with tiny bit of foresight they could have made that include a new image standard which everyone would then have as well with essentially no additional support needed. Instead we now have to convince all browser vendors and more to support yet another image format that has no widespread adoption.
Those other formats weren't developed by huge corporations who can force it into browsers through pure hegemony. That's probably a factor.