Try https://www.reddit.com/settings/profile

There are so many uneven areas of Reddit where WebP doesn't work. Old reddit, profile support, mod tools, etc.

I'm convinced that this is because of the prevalent MVP culture in modern software engineering. Instead of holistically looking at a new feature request such as "support webp images" we break it down into parts (e.g. "serve webp" "accept webp upload here" "accept webp upload there") and then we call it a MVP when only the highest priority items are done.