when a guy or a girl turns 18 why do they have to leave their home away from parents to live their own life ?

i saw random youtube videowhere a girl who's 18 says she's homeless and her own parents says she's not welcomed into her own home ? i was so terrified to see that.. does it happen to all the children who turns 18? or is it by a choice that when you turn 18 you can leave the house? i know 18 sounds adult but the truth is it's not. comeon 18 is a teenage where you just start to know so many things in the world.