Instagram hasn't even been primarily or even secondarily about photos for a long time. Indeed trying to "just" upload a photo is made super inconvenient these days.
Unless you're uploading memes you've downloaded from elsewhere, this strictly isn't true. I'd consider myself an Instagram power user and the only thing that I and all the people I interact with is photos and videos. None of those are webp, or would have been worthwhile to save as webp as an intermediate format.
Tangentially related but Instagram is really the worst plattform for photos. I don't understand why they crop and downsize (!) pictures. Not even Twitter does this, it's unironically a better photo plattform.