Why does your period have to happen to girls and not guys?

Menstruation occurs in both girls and guys - anyone who has a uterus, ovaries, and a vagina will menstruate - yes some men have those body parts and thus menstruate, this is why when talking about people who menstruate we say 'menstruators'.

Menstruation is a result of the menstrual cycle; your body releases an egg and prepares itself for possible pregnancy, if pregnancy doesn't occur then the uterine lining that built itself up during your cycle in preparation for pregnancy is shed and readies itself for the process happening all over again during the next menstrual cycle. Without the menstrual cycle we wouldn't exist.