Sure, but PBS member stations also function as incubators, in addition to providing a platform. They provide (along with underwriting from 3rd party charitable institutions) artists/intellectuals/entertainers the upfront capital to produce their programming. YouTube isn't going to provide anyone with money upfront to make a show unless they already have a massive following. Mr. Rogers Neighborhood wouldn't exist if WQED hadn't taken a chance on a couple of 20 something's letting them produce Children's Corner in 1958.

It's certainly possible that's less necessary nowadays, given how cheap filming and creating video content is nowadays, but it's worth considering.