Aside from the answers you’ve already received, there’s another one:
People find it mildly annoying when your uninstaller leaves some things behind. People get very angry when you uninstaller deletes things it should not delete.
So uninstallers are often written to be conservative, and only delete the things it can be absolutely sure the installer put on the disk. If it comes across a file that it’s not absolutely sure about, it just leaves it. Patches and similar can render the files not quite the same as the installed version, so to be safe they don’t delete it.
There’s multiple examples of uninstallers not being that paranoid, and then rendering the computer unbootable when they deleted something they should not have.
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