Are you a developer?
If yes to both, then there are apparently developers -- at least one developer! -- who decide what to develop.
And if you are capable of doing that, then why should every other developer in the world be incapable of doing it?
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.
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.