Back in time before they enforced the WFF and ruined third party stores, I suspected them from protectionism. Like it would give here for free something, that somebody else is selling as premium. But since WFF ruined such business models, I think they just do not care to give people such “mighty” option in a user friendly program, using same excuse for years, that it was out of scope of “mere designers tool”. Who would ever wanted to learn other (proper) developer tools, if everything were possible with this one?
Here we had nice discussion about user variables some years ago in feature request thread