Tech companies hire PR firms to do their promotion. They don't give the PR firm a cut of all the revenues the company makes, nor do they give them equity necessarily (it does happen but its not the rule).

The 'labels' have no reason to exist, there is however a great market for Artist PR firms. We need that first disruptive one though, where an artist 'signs' with them meaning that they do the promotion but they don't own the music copyright. And as it is a new business model for the music industry it will take some time to work out.