why do girls fake pregnancies?!!!??

so this girl in my class , i'm a senior in high school, told me that she's pregnant like 4 months ago and now when we started school again i seen her and she's not even pregnant anymore she said .. I'm friends wit her brother so i told her brother is her and the baby okay? and then he's like ummm she's not pregnant and never was? 😂😂😂wow what a liar ass girl... Why do girls do that and lie?