I don't like the gate keeping either but so called "entrepreneurs" would just go around washing people. The point is that you have enough money so no-one cares that the space mining company you invested in only hires website designers. Pumping penny stocks is illegal, but when it wasn't, salespeople were washing people.

It could also merely be convenience - as a startup you want to raise a few millions or whatever, and you want it all to come from a few sources, not too many. If you opened this up to an average investor, it will be a headache consolidating all of them. Also, there are offerings by banks for wealthy clients who can invest in the banks' PE or VC arm, this allows access to the clients, and since it's one bank providing liquidity the people obtaining it don't have to manage anything.