I think it's more than just the look. Surely 2D figures look better due to the lack of details, so they are more or less forms that are too perfect to exist in real life and more diverse either, since you can have everything, from catgirl to alien robot.
But I think that the main reason is something else and psychology related. Since a 2D character doesn't exist they cannot cause you stress or be around you when you want to be alone. They automatically switch off once you stop thinking of them so they are never work like real people you have to entertain or be around with.
Even more than that, whenever you visualize a scene with them you kinda are them, or controlling them. Sure, you play by the rules and let them act within a certain frame of personality traits the canon gives you. But even then it's still just an interpretation you are directing as God/puppeteer. So you will never feel insecure or be not aware of your waifu/husbando's thoughts since you create their thoughts.
It's probably some autism thing but I think that the love for 2D is genuinely different from the love humans have for other humans. Can only talk about me though but the main reason for why I will never care or get close to real people is that I don't know their thoughts. At the very best I can vaguely guess their mood but that's not enough for me to genuinely relate.