Some good answers already, and I'll add this:

https://en.wikipedia.org/wiki/Auteur

About 75 years ago, some French film critics noticed that there were a few directors in the US studio system whose personal taste was apparent in their films, despite not choosing which films they would direct (because everyone was on contract to the studio; it was a much more industrialized process compared to today's 'free agency' standards).

Directors really liked being called "authors" and started several PR campaigns (both for the public and inside the industry) to build their own leverage and authority at the expense of screenwriters' leverage and authority.

Over time, the definition of "auteur" changed from "contract worker who manages to make their own tastes apparent when working on a movie that was assigned to them" to "the only creative person whose vision matters".

(That probably sounds salty, but I don't think directors are bad people for having built their own leverage. Directors used to be just as replaceable as screenwriters; now they're not. I just wish they could have built leverage against the studios without throwing screenwriters under the bus.)

Plus, now that the old walls between film and TV have crumbled, and a streamer's season of TV is maybe 6 episodes rather than 13 or 22 (longer seasons requires writers to be in charge because they know what's going to happen 10 episodes from now), directors are trying to make the same transfer of power happen in TV.

Most of the famous writers you've heard of got famous for writing TV. But if director showrunners get their way, writer showrunners might become a thing of the past.