This does exist. It's called BlackBird. I think there's a couple other attempts at this market as well. It's been a bit controversial and there have been some discussions of how legal it is but it seems to still be up and running.
I think the illegal-per-FAA 'Uber with pilots' part got shut down and now they just link their scheduling system with regular part 135/charter operations.
Interesting. Looks like that news is about a month old. They had been trying to get away with that idea of you rent the plane and then you hire a pilot theory for long enough that I thought maybe they’d get away with it.