Good luck handling software development, talking to customers, gathering requirements, aligning with other teams, doing marketing, writing budget requests - all by yourself.
pretty much what most PMs don't do exactly, they just take what managment says verbatium and just throw screenshots on top of another screenshot together without even bothering to properly account for most common states, research? user insights? etc.. it always goes like this: we made change x, our y metric increased. Waste of space.