How so? With Darkseid, there's a ton of stuff I can do. I haven't even touched on New Genesis, yet, and there's a ton of stuff I can do there.
Same withg most other cosmic baddies. You think Galactus is just like "Oh wait, it's Tuesday, maybe I should check out what's going on at Earth today!" There's other plots and storylines they can concoct and interactions. Personally, I really want someone to pick up, say, Thanos because that would give me a PC to interact with who isn't based on Earth. The big cosmic villains allow for lots of development and plots based on them, whereas I really get tired of the "street level" villains. Partly because they get much more play than the big bads, and because they get beaten so much more often. It's like, Doc Ock shows up with some new toys, gets the crap beaten out of him by Spidey, and goes away until he can back with yet another new toy and get the crap beaten out of him by Spider-Man. Yawn. At least with guys like Galactus and Darkseid, it forces the heroes to deal with the fact that they will in all likelihood lose against these guys in a physical confrontation, so it forces lateral thinking on the part of the good guys.