One word: objectivity.
At the risk of sounding like Mr. Know It All…
Having done that work for many years, the answer is simple: You can’t USE the software -and- DESIGN it. There just aren’t enough hours in the day. UX designers (like all of us) fall in love with an idea and then are loathe to discard it unless they are YELLED at by others who actually have to live with it 10hrs a day. It’s like trying to master your own record… it’s usually a fool’s errand. Most of us just can’t step outside ourselves and hear our creation the way others do.