Linux

There's not much going on here except for familiarity and some gaps in your knowledge of computers as a whole.

As others have said, Gimp saves files in a project file format that preserves things like layers and edits that aren't possible with png or jpg. It's the equivalent of Microsoft Word using .docx vs exporting as PDF. You can't save your Word doc as a PDF, delete the docx file, and then expect to be able to open it back up in Word and continue editing. If someone needs a PDF and you keep sending them docx files because you don't understand the difference, that's an education problem, not a technical one.

Something like zip vs tarballs...you're really only seeing one small part of the world here. "We don't need the user experience to be bad" is actually a good observation, but you don't have a full understanding of what constitutes the user experience. As an example, a tgz file stores Unix file attributes, and zip doesn't. So if I'm not sending the file to a Windows user, zip makes for an incredibly bad user experience. If I zip up a directory full of files with specific owners and permissions, when I unzip it later, I'll have lost that information. If the "default" compression for Linux (and that's kind of not a sensible concept, but let's go with it) were zip, then the vast majority of Linux users would have to exactly what you don't think people should have to do -- go change the defaults after they install applications. Most Linux users don't want there to be a baked in choice that will be wrong half the time. I want to make a zip file to send to Windows and a tarball to use locally, and I want to make that choice consciously based on what problem I'm trying to solve.