It's just the default target. For backend stuff, you're right that there's really very little true value. But client software just has a lot of complexity. If you run binaries from wherever, they're probably built and tested on Ubuntu first. Like, your vendors won't support distros officially, install scripts and AppImage integrations are going to tend to assume stuff. Driver/runtime integration for things like binary GPU stacks are going to document Ubuntu first. If someone makes an assumption about Python version (a personal peeve -- LOTS of python software ends up lagging the tip, it's not a super compatible ecosystem), it's going to be that you're running Ubuntu LTS version "one before latest", etc...

Are those fatal flaws? No. But lots of people just don't want to deal with it. And there's a market for that.