In fact some pros do upload to social media, but they also want to make money off their work and not spread it around for free.
As I said before, in what other profession are amateurs allowed to be present, and offering their services while the pro is working.
I've had people try to bring along a friend with a camera, to outdoor family portraits I was doing. Naturally, I made them stand far away when I was working. But as I was leaving, I saw them do my setups with their friend, so that they could use those pictures for other things. Those other things are what can cut into my profits.