Whatever the reason, we're surely becoming culturally poorer.
Screenwriters would seem to be critically important, but writing is hard, often slow and converts much less recognition than other roles in the industry. Further, there are innumerable points in the production chain in which the (necessary!) decisions of other crew members can alter/dilute/confuse the intentions of the screenwriter.
I can name Levinson and Link. Ronald D. Moore also seemed pretty great. Some writers are talented enough that they can transcend the obscurity granted by their profession, and achieve "cult status" within a small niche who cares about these things.
But frankly, it seems many writers are simply not good. Most TV show nowadays consist of central characters doing dumb things, and needing an episode or two to dig themselves out, acting as filler. Episodic formats tend to promote lazy writing and overreliance on tropes. And it seems to me that most writers watch more TV shows than they read books, given how homogeneous the average popular TV show can be.
That's a quite accurate observation. Even for the largest budgets people tasked with story telling are almost always the weakest element. Who makes them? What kind of education do they undergo to be this wrong about reality and storytelling? Are they such small and in-bred group that they are all so wrong in such similar ways?
This reminds me of another weird thing. In games, various NPCs often say the same dialog line just in different voices. So the bottleneck is writing few words? Not hiring voice talent, not recording it in the studio and postprocessing it? But writing few unique words less is where you get your savings?