Hmm. You raise a lot of interesting questions. Ones that I've never thought of and don't have full confidence in answering but I'll try. Albeit, out of the order in the OP.
First off, humans are a social species. As much as a lot of us don't want to admit. We want companionship whether it be with friends or with people we want more with. We want at least one other person to understand us on a deeper level than what we usually show to strangers. And with interacting with many types of people in whatever manner, we see things from different perspectives and change.
Personally, I think we are over-saturated with romance in popular culture. I also think that romance shouldn't be in everything. It all depends on what kind of story the author wants to tell. You can tell a complete story and have your protagonist(s) grow and develop without having a love interest involved and just be surrounded by really good friends. People can go their whole lives without being in romantic relationships and still function as normal human beings.
Romance is relatable in a story because people pursue it. And with romance brings drama and people like drama. It's exciting and draws people in. It makes you wonder what's gonna happen next. That is if it's competently or interestingly written.
But more often than not, authors tend to diddly up romances. Whether it just be incompetence or lack of experience or many other factors and/or combinations of them. A lot of them tend to write romances as if this two of people
have to be together or they'll never be happy. Or sometimes they just get together because that's what's expected. Or just to check off that genre box. Or the characters just plain don't have lives without their romance endeavors. Or they just want to write their self-insert fantasies.
Here's a video that I like to watch about romance in anime that goes into more details. Granted, it's more based on his preferences, but I still think it's worth a watch.
One of my favorite games is Persona 4. You can play through the whole game without getting with any of the female characters you interact with. The MC doesn't really change because he's a blank slate for the player to project themselves on, but the females still develop. With his help, they figure out their own problems and what they want to do with the rest of their lives. Yes, they show signs of romantic feelings towards the MC, but in the end they still go towards the same goal with or without you acting on them. And you can still enjoy the game for what it is: a JRPG that delves into the literal psyches of its various cast of characters with themes of friendship, sexuality, self-reflection, acceptance, maturing, and a slew of others that I'm probably forgetting while trying to solve a supernatural murder mystery.
But I would like to see more non-hetero pairings in stories. And not in a marketable, shoved in your face, "Hey! These two are gay! Ain't that cool and unconventional?!" Just two people that enjoy each others company. I would even like to say expose it to a younger audience. Show it's okay to like people of the same gender, or someone in between, or someone that belongs to none of the above romantically. And nowadays we're getting more of that and I like it.