Or they just can't find any decent screenwriters around (I honestly doubt).
There are no doubt plenty of good screenwriters, but as I've been saying, they have no power in the feature industry. A typical film may hire up to a dozen or so writers, mostly uncredited; have each individual or team write a different script; then cut and paste together the bits of multiple scripts that the director and producers liked; then have maybe a different writer tie the fragments together into a more or less coherent script, which is constantly rewritten during production at the whim of the director, producers, actors, etc., and then gets further transformed in editing and reshoots. A single writer, even a credited one, has no more quality control over the process than a single assembly line worker in a factory.
The problem with the feature industry is that directors have all the power and many directors don't know what good writing is, while assuming they do. I read once that the problem is with the film school culture that produced a lot of modern directors, which stresses visuals and style and technique but doesn't stress the basics of writing and storytelling. A lot of modern directors built their careers on music videos and commercials, so long-form storytelling wasn't one of the basics they learned. Which is why there are so many stylish, gorgeous-looking, well-acted and well-made movies whose stories are incoherent garbage.
Of course, you're probably right that the fundamental problem is the studio's attitude. Even with all the strikes against writers, there are still good, well-written films out there, when the studios and filmmakers are willing to put care into that process. But it seems like the people in charge of the Sony Marvel Universe (or whatever its current name is) are just trying to churn out IP that they expect will automatically sell tickets because it's Marvel. Or else they're just trying to hold onto that lucrative IP by keeping it in production, the same thing that motivated them to keep rebooting Spider-Man and Fox to keep making X-Men movies. (For an example of this principle taken to its pettiest extreme:
https://www.facebook.com/ryanestradadotcom/posts/10167532089040486 )
Which is part of why if you've seen the trailer you've seen most of the in-costume action that's in the movie.
The sad thing is, those are actually decent-looking superhero costumes.