I don't think they're exactly necessary. Though I do think they've come to be expected in most long-winded stories, no matter the medium. Just one more thing to help the audience get invested I suppose. Plus its only natural to develop strong bonds with people you're saving the world/universe with, and to get to know them way better than they probably ever wanted to before. I'm inclined to believe there would inevitably be romance between members of a real life world-saving team if it were made up of a bunch of single awesome people. I guess the only question really is, why not? I'm hard pressed to think of many RPGs that didn't have any romantic sub-plots. For some reason the only examples I can really come up with are either really old, or strategy RPGs. But there are a lot where you at least have the option to not pursue romantic interests. Which I don't think I'd have a problem with. But it would be jarring having a long in depth story without any romance. It's about as foreign as having an elderly person as a protagonist, and I'd want just as much to see where that sort of thing could go