Why do dekstop environments make custom xdg-open replacements?

Related to this answer on my previous question.

So, XDG can handle it on its own:

• It can manage default applications - xdg-mime


• It can open file with associated application - xdg-open.

Why do desktop environments make their own replacements for xdg-open? Like gvfs-open, kde-open, or exo-open,...

Isn't xdg-open enough? If not, what does it lack?