Because movies and TV don't just make themselves. Paramount/CBS might want Star Trek, but who is going to make it for them? Writers might be able to write scripts, but making TV and movies is much more involved than that. Who hires the writers? Who gives direction to the writers?
JJ Abrams supposedly became friends with Kurtzman and Orci on the set of Alias, so JJ taught them how a show is put together. He taught them the basics of how to "produce". And now, sometimes K&O write, and sometimes they produce. When K&O produce, their track record is shockingly better than their writing track record.
Many producers don't have a background in writing, so when they boss the writers around (which is part of their job) the writing suffers. K&O do have a background in writing (even if their writing's not so good), so when they boss the writers around, they're better able to know when to back off, or when to have a light touch, or how to contribute something that is actually helpful and not counterproductive. Which lends to the rumor that when K&O produce, they're popular with the writers on the production.
K&O (after writing Michael Bay's dogshit Transformers 2) produced Transformers Prime, which is debatably the best Transformers cartoon ever. When they stepped down as producers and walked away, the show was cancelled. When it came back to life, it was a hollow shell of it's former self, barely worth anything. Producing is a real job, requiring real effort, and Kurtzman seems to be good at it.
It's like, John Wick was directed by a stuntman, which is why all the stunts in those movies are great. The director isn't doing any of the stunts, but he understands stunts, so his relationship with the stuntmen on the set is great, and it shows.
