Uh, because flirting is about attention, not necessarily about sex, and certainly not about devoting time and energy to multiple healthy, loving relationships.
I'm still confused as to what's going on here. If YOU'RE the one interested in poly then it's about YOU and YOUR relationships. Not hers. If you want more than one girlfriend at a time and she's not ok with that, then it's up to you to decide which you want more- her or the multiple relationships. But she shouldn't be "flirting with other guys to make you happy" nor should you be trying to push her into having multiple relationships. You do sound very confused, but she does not. You gave her the information, she told you her boundaries. You need to respect her choices and decisions.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack