One thing that I can't quite figure out, is that in comic books (Alan Moore, Neil Gaiman) and roleplaying games (John Tynes, Dennis Detwiller, Greg Stolze), the horror genres are where you go to see all the smart writing, but in films, while there's a few sharp pieces, there's also a lot of sloppy trash.
Moreover, it's not like filmmakers couldn't steal these guys away from their current job for the length of time it takes to make a film -- RPGs pay practically nothing and film budgets are such that even successful comic writers could be lured.
I have a theory on this, though. The success of the horror genre in film is what has lowered its average quality.
Eventually every small production company that's starting out figures that they ought to make a quick horror film or two to help drum up the cash for their real movies. So you have some people who actually don't like horror making decisions about how horror should be made.
Once they set their mind to it they'll convince themselves that they're horror fans and understand the horror genre because they liked Aliens and The Exorcist. I liked When Harry Met Sally, but I can guarantee you that I do not understand the romantic comedy genre.
In some cases the project will take the form of "we want to make sure that everyone knows that we're just having fun here, not taking it seriously, and you should look out for our real stuff down the line". That often results in campy horror, which in the 80s could make decent money, but today doesn't even warrant a theatrical distribution. And it sucked in the 80s too.
Otherwise, it'll just result in a series of mediocre decisions -- often partially saved nowadays by higher budgets, thus allowing talented cinematographers and production designers to at least make the visuals powerful.
Despite this, even mediocre horror films can turn a tidy profit. And this is another reason the average quality of horror films is low.
If you've put together a $1-3 million budget drama about the trials of family life or the emptiness of the human experience, it has to be fantastic to make it onto more than three screens.
But you do the same with a horror film, and as long as it doesn't actively stink, they'll toss it out onto at least a few hundred or a thousand screens and make some cash. Which, of course, drives more people who have no interest in horror to try their hand at making horror movies.
And the final link in the chain is me, and people like me. Horror fans desperate for the kinds of stories that they won't show on television, and who enjoy seeing a film in the theater -- so we're willing to buy a ticket to even the mediocre ones because they're all we got, and even a mid-range film usually includes a few excellent moments (which is often what makes it a mid-range film).