The format itself is perfectly fine, it’s just that most software doesn’t work with formats made in the 21st century

Literally, you answered your own question. From the user end, unsupported file types of any frequently shared format are garbage. No one cares on the user end about server space. They care about sharing a funny image. They don’t care about 2 extra ms of load speed. They want shit to just work.

It’s the same reason Open Office sucks. You can’t rely on it to just work. As much as dev’s hate it (myself too), reliability is king. Webp fails this measure, badly.