Great question! People use drugs for lots of different reasons, such as peer pressure, boredom, rebellion, curiosity, and so on. People can know drugs are bad for them, but they may still choose to take the risk, while others may not be aware of all the risks. Then, once they’re hooked, it can be very difficult to stop. Check out one of NIDA's Drug and Health Blog posts about peer pressure here: https://teens.drugabuse.gov/blog/post/i-wish-that-i-could-be-like-the-cool-kids
There is also a lot of information about the health effects of drugs on NIDA’s teen site here: https://teens.drugabuse.gov/drug-facts.