That's why Lew Wasserman's innovation in building the modern talent agency at MCA / packaging films was the death of the former studio system, and genesis of the contemporary star system.
You don't really explain why. The why is that before MCA, the studios directly employed actors, directors, writers. They were vertically integrated and held all the power. After MCA and CAA, the talent broke out and negotiated with studios on a project basis. And, instead of getting salary, they started getting a percentage of gross office revenue. They sort of went from employee to startup co-founder because they owned a stake of the film. The power dynamics since then are completely different.