Why romance sidequests are more important than the main quest...to players, specifically. No, it's not because all gamers are lonely nerds desperately yearning for contact with the opposite sex, but never getting any. More important is the fact that in most RPGs, the main quest is usually given to you, while a romance sidequest is a mid-to-long-term commitment you take upon yourself — the difference in terms of game design is immense. In the case of the main storyline, the player's decision to engage with it is implicit in their decision to play the game at all; in other words, you don't choose which embedded storyline to follow, because usually there is only one embedded storyline. And although you can often personalize some superficial details of the Player Character's journey (like their looks and morals), you cannot set its destination, so when you reach it after 40-80 hours of gameplay, regardless how much you empathize and associate yourself with your character, it is still a goal that they (or, more accurately, the developers/writers) chose, not you.
By contrast, in the case of romance sidequests, both the decision, the effort, and the ultimate pay-off all lie with the player. As a side-quest, the player makes a conscious decision to engage with it, especially if effort is required (such as locating a potential love interest in the game world) even to just access it; even more important is the decision of which love interest to pursue, which many players make long before starting a new game thanks to online bachelor(ette) lists existing for every game. By making the player choose their love interest early in the game, then making them work on their relationship for 40-80h, before finally resolving the Romance Arc in the endgame, the devs give a new meaning to your playing of the game. You no longer play just to learn how the story that they want to tell you ends, but also to discover how the relationship between a character whom you created and one whom you chose develops. Even if said relationship is also an embedded narrative written by the devs, the fact that you had to pass on other available romance arcs to experience it makes the delayed satisfaction from completing it just so much better.