The people that go to them don't expect much and hence are rarely disappointed
Actually, prior to X-Men, we were regularly disappointed. Because everyone who tried to make a comic-based movie did a terrible job prior to that. There's almost not a single comic-based movie before this which treated the material well and didn't devolve into some corny parody,
Are they escapism and popcorn cinema? Absolutely they are.
But, what you can't argue with is the bottom line -- they make money. Lots and lots of money. When X-Men came out on DVD, the sales of the DVD were higher than the highest grossing films in the box office. That was the first time sales of a DVD had done that, and suddenly people stood up and took notice.
Disney bought Marvel for something like $4 billion dollars. I believe the Iron man films alone have brought in something like $4 billion dollars, and that's possibly before we hit the merchandising.
So, you may not like them (and nobody says you have to), but there's really no denying that the Marvel properties which have been turned into film since X-Men have generated huge amounts of money, have been seen by tons of people, and have even more films (and money) in the pipeline.
DC is hoping they can cash in on the action, but they may not have as many properties as people relate to, and if they don't have a "big vision" kind of deal where someone who knows the material keeps it such that the fans still watch it.
If they carve it up, do a bunch of things which don't go according to canon, or generally do a half job and expect to just roll in the money, they could be seriously disappointed.
Marvel has been smart, they know the rules and stories of their characters, and have entrusted it in the hands of people who actually know the material. Which means the people who want to see them don't find themselves halfway through a film going "no, that's not right".
Contrast this with the Spider Man series, which is a Marvel property but has been in the hands of Sony. They're on their second reboot of the damned thing. We don't want yet another Peter Parker origin story because you don't want to pay the actor. If that's all you have, just stop.
So, "pre sold to comic fans" isn't a gimme. If DC just acts all cynical and "give me the money", they might find they've made crappy films that nobody has any interest in seeing. Think Dare Devil and Electra.
The proof is in the pudding, and in the revenues. Just jumping on the comic book movie isn't a guarantee of anything.