I could save my photos as BMPs like early digital cameras did but that doesn't make it practical or reasonable. My camera takes pictures as RAW or HEIF files. Why would I save my photos to a primarily lossy codec that's optimized and designed for distribution rather than preserving fidelity?
We used to do this with JPEG, in fact. And that's why many pictures on Facebook from pre-2018 or so all have a distinctive grainy look. It's artifacts on top of artifacts. Storage on phones isn't tight anymore, we don't need to store photos in a format meant to minimize bytes at the expense of quality.
There's more on Instagram than photos. Lotta meme pages, lot of people just uploading random screenshots and photos they downloaded that have been turned over a million times. Heck, all it takes is someone downloading their own photo from SocialMediaX to reupload on SocialMediaY, or just uploading a the WebP that they exported for their website.
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.
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.
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.
