Why are people this obsessed over another human?

Because enormous sums of money are spent marketing this individual as a form of paid parasocial relationship. You get (fake) emails from her. You get (fake) social media interactions with her. You get a deluge of media telling you that she's available and interested and looking to meet a guy just like you. You spend hours in line to get tickets to a sold out show. You wait with baited breath for every new media release, which is inevitably a song or movie or other material about her falling in love with a stranger who follows her from afar. This, combined with the endless peer pressure on other women to become this iconic individual.

Idol culture is the social equivalent of playing the lottery. And finding out your idol is "taken" is like hearing you got sold a ticket to a prize that's already been awarded to someone else.