That they are doing it doesn't mean they want it this way. Ubuntu calls its half yearly releases unstable (or used to), which scares users off. Upgrading is also hard on ubuntu (compared to arch, where the set of installed packages stays (apart from renaming and similar) the same). And for the software where they need it, the users already use ppas, probably. And then there are the users who almost only use a browser. And there are enterprise users who don't upgrade because not all of their software works on new ubuntu.