That's one reason. Another reason is that while some effort was spent on writing this spec, apparently (?) almost no effort was spent on promoting/enforcing it: there is another top-level comment in this thread from a 10-year Linux application developer that says they've learned about XDG from this very post.

And indeed, there are lots of tremendously popular apps out there (Slack, for instance) that use e.g. $HOME/Downloads as a default download directory instead of $(xdg-user-dir DOWNLOAD), and most users don't mind.