because we segregate based on gender not based on how tall someone is etc.. this was answered already in the first few posts
she's an intersex athlete competing in a woman's event, as a result she has a huge advantage due to her internal testes and high testosterone levels (which is one of the main reasons for male athletes having better performance in athletic events than female athletes) - the compromise is to allow intersex (or indeed trans) people to compete provided they either artificially lower their testosterone or have their testes removed (personally I think this is a big of a fudge as they've still got other advantages as a result of being essentially males without penises who've been brought up and identify as females)
indeed, she was basically born as a male baby that didn't develop a penis properly as a result of a birth defect - she'll still have a Y chromosome, a male figure, strength etc.. it's just she has a vagina with no ovaries and has been brought up identifying as a girl