the girl that's straight but still flirts with you

WHY, please tell me why, WHY do my straight friends flirt with me?? "haha, my dear friend, what if i kissed you right now? haha, but im just kidding, its all fun and jokes of course" WHY the hell do they always do this ?? and the worst part is, you can never ever be sure that theyre actually joking, because so many girls will do this, and then one of them turns out to actually like girls, and then she wants to date you, and then youre never sure anymore, why do they do this, this one girl genuinely wrote me a full VICTORIAN LOVE LETTER for a joke? like what the hell? and i always tell them hey, dont take this too far, you know i like girls, and i WILL get flustered and even maybe catch feelings! and they tell you haha, dont worry, i wont take it too far, and then they DO, and ill say, well, hey, you just KISSED me, im kinda flustered, so, you wanna like, go out, and shell say, "hey, no, no, no, im 100% straight, no, were just friends" so WHAT THE HELL is up with those girls?