That sounds logical, but on most operating systems these days the extension is hidden, and/or you need to go through some hoops to change it. So I would think that most people who think that wouldn’t even know how to change it.
But more importantly, where do those PNGs come from in the first place? Sure, some are clearly screenshots such as of IG or TT, but there are tons of large PNG images that are clearly photos from cameras that someone just took and resaved as png (and later, or someone else, then renamed to jpg).
I could understand that happening occasionally for a bunch of reasons, but I’ve encountered this so many times, it’s pretty bizarre.
Btw it’s something you might not even notice if you aren’t using e.g. an image viewer that uses a different icon or background based on actual image type.