> Genuinely curious, why does it matter if a relationship is real if the person is happy and mentally healthy?

It's like having an imaginary girlfriend in your head. If it makes that person happy then why do other people feel bad for the guy? Because although that guy may look happy, and maybe is happy right now, most people don't really believe that's a recipe for happiness in the long term. People see somebody on a path to loneliness and pity them for it.