From a psychological perspective, whether or not you actually have a chance with her in the literal sense is not the concern. It's in your instincts that you find women in relationships undesirable, and you don't feel fun when watching her. If the chuuba is single or just has a girlfriend, your genes tell you this is desirable and it will make you happy. That's it.

Your enjoyment of watching anything is bound by your neurological makeup, and no matter how you try to reason with yourself that it's still supposed to be "fun" even if she has a boyfriend, your brain just won't agree with you. The cope that you will never fuck her anyway just won't fool people with above average IQ and normally developed brains.

This is not to mention these women are given the easiest job in the world where they make millions just for playing video games. All fans ask for is to remain pure and celibate; this is not overrequested. And when they still choose to get boyfriends or even marry like it's nothing... shows a lot about their character.