Why are we not able to switch career branches?

Using University degrees to change branches should work for the Romance career, just like any other career, as long as it has University degrees that provide advanced placement in those branches. Obviously it won't work if there aren't any degrees associated with the career.

I don't have Lovestruck, so I can't test it for you. You'll have to check for yoursefl whether any of the majors provide advanced placement in the Romance career.