Why not take drugs?
Why not have sex?
Why not drink?
There was no particular reason to do those things, but there was no reason not to. Then there are the people around you, people who seem to like you and enjoy having you around, older than you. They are drinking, getting high, getting laid, and seem no worse off for it, so when they offer to share, why would you say no?
There may be something in your life that got you to these people in the first place, something you want to avoid, something that means you would rather be out than at home, something people your own age don't get, so you avoid them. But these older people, they know pain, they make yours seem survivable, they teach you how to laugh at it.
(You ignore the things that maybe you should worry about, that you and maybe another couple of girls are at least a decade younger than anyone else in the room.)
The idea that this is exploitative is laughable, what the hell could these older cooler people possibly want from you? You don't have anything to offer (your previous life experience has already taught you that) People tell you that you are being abused, but you already know what abuse is like, and this isn't it. When you are being abused you don't get looked after, and made to feel cared about. Abusers don't invite you to share their stuff and give you somewhere to stay when you can't/don't want to go home. They don't introduce you to new music and explain how to take drugs safely.
They teach you more about how to grow up than you ever learn at home. When home say they just want to protect you, you think of all the ways they failed to do that in the past, what the hell do they know about protecting you. You are better off with your friends.