Because every other developer in the world screeches and retreats to their lair when you so much as mention attending a single meeting, let alone the number of meetings and conversations with clients and leadership I have to keep up with what we need to build next.
Additionally, for a dev to directly learn what to build, they’d need to work two full time jobs. Most prefer to do just the one job.
Oh, I see: The fact that you can do that doesn't mean that anyone else could, it only means that you're so much better than anyone else. OK, thanks, gotcha, good to know.
It's just two separate jobs, why would anyone sign on to doing two jobs when they can specialize in one or the other, unless they're in a situation like mine, which nearly nobody is.