cause your parents have to be your parents first and your friend second. you have friends that can be your friends. without any sort of direction, kid would be shit out of a luck. how would it make you feel if you were a parent, trying to protect your child, and all they wanted was for you to leave them alone and feed them, put a roof over their head, and give them money. plus, a lot of the time little things progress into bigger things. you'd hope that your parents would intervene if shit got real, but it's better to prevent something from going overboard before it actually does.
i don't condone parents completely forcing their own will on you, but as a guide? you've got to be strong enough to be able to endure a reasonable amount of lecturing. expecting your parents to back right off and let their teenage children to just do whatever they want, to me, is just insane. we need guidance. that's why we have teachers in schools and not just books on shelves that are there for us to pick up and learn whatever the hell we want to at our own pace.
and yes, we all fuck up, and ultimately we all need to learn from it, and we all need to choose our own paths. but it doesn't hurt to have someone looking out for you and passing their experiences on. if there was a cliff at the end of the road i was driving down, i wouldn't want people to just say 'oh well, everyone's going to drive off of it, gotta let it happen'. i think i'd want someone to step up and tell me not to drive down that fuckin road. and that's what it's all about. lots of the time you aren't aware of the path that you're on, but you can sure see as an outsider where people are headed.
i think the whole 'apple doesn't fall far from the tree' thing plays a part in out parents' overprotectiveness too. if your parents are living with regrets, lord knows they don't want to see their child grow up a shadow of their former selves having the same regrets, so they try a little harder to point them in a different direction in an attempt to give them a better life. whether it works or not really doesn't matter - the end doesn't justify the means - but they are actively trying to give you a better life. to call them out for controlling you is extremely selfish. lets put it this way, there's a reason the world isn't run by teenagers. we have to acknowledge that they aren't exactly the best decision makers. so if your parents want to get on your case about your drug habits, the friends you hang around with, and your shitty grades. so what? appreciate that you have parents who care and have seen what happens to the people who grow up prioritizing the wrong things.