Why do I feel like the only way to fit in is to do drugs or drink alcohol?

Because you have the natural urge to belong somewhere. The question is, what you look for in fitting in? Is it comfort, safety, acceptance or intimacy? Those are all very natural feelings and people that experience them usually don't need to use drugs as a tool to get those feelings. Perhaps learn from people around you that don't need to do drugs to feel that way. Study them, or ask them directly, or hang around with them for a change too. Perhaps your need to fit in is natural, perhaps you just don't know about other ways to fit in without sacrificing your health or integrity...

I have found it's easier to fit in with the more simple minded people who participate in these things.

A lot of times, people think that doing drugs or drinking alcohol will make them fit in, but It is not the only way. Even if you have friends who drink or do drugs, you do not have to. You have a choice on whether or not you want to do that to yourself and your body. If people are pressuring you to do drugs and drink alcohol, then maybe they might not be the right people for you to hang out with. It may seem like they are, but there are plenty of people who fit in with their friends who do not drink or do alcohol. You just need to be yourself and see what you want to do. Don't let others decide for you just because you want to "fit in'.