My main idea for video games is that they transport me to another world for a time. I'm married in real life and know the benefits and downsides of, GASP, actual relationships. Like Rincewind to his fellow mages, I've even touched women putside of marriage so I'm different from 60% of my fellow gamers. So, it's not a weird substitute or anything but it is a point of the fantasy/story. I'd rather interact with fictional characters and have a story arc with them than blow away my 30000th zombie. Romance is an easy and natural form of human relationship, which can enrich a story a great deal if done right.
Now, mind you, the majority of video games never really do advance beyond "rescue the princess" which is a shame. Also, almost no characters are in relationships period beyond generic UST. Chris Redfield is now his late forties and as far as we know, he's never been on a date with any of his partners. Adam Jensen is kind of stunning as he actually did have a long-standing relationship with a woman before the game (but not after or during) which is almost unprecedented.
I've mentioned in my Wolfenstien: The New Order review the weirdest thing about that game of Nazi murder is BJ gets in a relationship with a character, they have sex, she doesn't die to make him angsty, and they don't have any relationship drama like him having to choose between her or killing Hitler. He doesn't even need to do any fetch quests or conversation topics to get her to sleep with him. No, they just like each other, and start having sex before deciding to get into a committed relationship. It's almost unprecedented--almost...mature.
It's funny we've got a dozen fathers in video games now but almost no husbands.