Hosting providers don't want these formats because they're more cpu-intensive for them to decode and transform (e.g. to make a thumbnail, or when you upload a 4K selfie to be cropped into a 256x256 circle avatar, or a 16 megapixel image of a receipt).

By contrast, if you let the user upload a JPEG or even a PNG, you might pay a slightly-higher cost of the incoming bandwidth, but then you (basically for free) transform the image to the small dimensions and format you actually need, and from there the bandwidth is barely a problem.

Sidenote: End users hate WEBP because nearly any software other than web browsers still don't support even reading it, let alone outputting it (and as you pointed out, anywhere that you "upload an image" they're not welcome either).

In my incredibly shoddy testing sample of 1, my current-macOS device does open AVIFs in Preview, so that's an improvement, at least!

I keep seeing people saying that nothing supports webp, but I've never had any issues opening it. What software do you use that cannot handle webp?