Yeah, but all that other material is produced outside of the games. The Sonic, Mario, etc. franchises don't inherently provide a universe that fulfills people's sexual appetites. All that comes from the fan side. DOA, meanwhile, appears to provide all that within the boundaries of the franchise. At least that's the impression I get. I'm not a DOA player myself so, full disclosure, I'm not an expert on the subject.
Bingo. I think that one thing about video games is that a game's mechanics and its conceit have to come simultaneously. You can't ask a player to stop his or her progress just for the sake of providing some fanservice (sexual or otherwise) and expect it not to kind of piss off the player. Think of the unskippable cutscene nonsense a lot of RPGs pull. DOA's conceit is perfectly in line with the tasks the player is asked to perform. It's a beach with beach stuff. All the fanservice is just an extrapolation of that concept. I think of it like Baywatch. Baywatch was actually a really clever show because of how natural it was. It didn't force its fanservice down your throat at the expense of the surrounding story. It was essentially an adventure show that just so happened to star lifeguards.