Actually, it is an editable format. It can be "locked" to theoretically stop people from editing a specific file, but most of the elements available in the format are vector. Of course there are raster elements that are purely a collection of pixels, those elements are not meant to be editable.
Some apps just decide to punt rather than deal with the vectors, so they stuff a raster image into the PDF and call it "good enough".

It's obvious that Adobe's "plan" for PDF went off the rails at some point, and they just started wedging features into it all half-assed. But the real problem is that Adobe clearly decided at some point to monetize the problems in their format rather than try to fix them.

And spreadsheets aren't supposed to be anything more than a convenient way to calculate things, but we know how that went.

There was no message, I just recently had to do a deep dive into our PDF code to fix something that is ridiculous and took way too long to work around (it wasn't a bug as much as it was PDF nonsense), so I am severely butthurt about PDF right now and venting about it whenever I get the chance.