But they are. They announced Conan a while ago, they have Star Wars, and then at NYCC just yesterday, they announced that they are bringing back a lot of old titles (granted, trademark retention is likely a motivating factor but some good talent is working on the books) which include horror, romance, war stories, western, etc.
https://www.bleedingcool.com/2018/10...ry-in-january/ It is unclear if they are oneshots or ongoings, but still.
Also, I'd argue that even though they take place in the MU, and sometimes superheroes appear or are otherwise involved, some of their shared universe books are other genres, such as Jessica Jones being a detective book for the most part. Do superpowers come into play? yes, of course, but in terms of feel and tropes used, it has so much of the detective genre in it, that fans of that genre can easily get into and enjoy it. Same with Thor and fantasy, or Guardians of the Galaxy and sci-fi and so on, even if they do sometimes change genres at times to be more or less superhero focused. That's one of the fun things about Marvel (and DC's) shared universe, it's diverse enough that a lot of different genres can be played with, even if they are all still technically superheroes. Superhero is more of an umbrella grouping, which can include many sub-genres. It's like in music saying something is 'metal' only goes so far, when you have everything from Cannibal Corpse to Nightwish, so you have death metal, gothic metal, black metal, folk metal, sludge metal, symphonic metal, prog metal, pirate metal, viking metal, thrash metal, and on and on, and sometimes crossovers of 2 or more of the sub-genres, and you can enjoy one sub-genre and not another. Superheroes is like that.