Some people are doing it because they're genuinely scared due to the propaganda. Some people are doing it because they think they're being logical. They are thinking "we won't go back to normal until enough people do this," and they don't believe risks of side effects are high. They might be right, but they just as well might not be. The truth is no one really knows for sure, and anyone who says they do is lying. It's a new thing, less than a year old. There are zero long term studies. It is not even "fully approved" by the FDA (for those who care about such things). It's only "approved for emergency use."

So basically some people are scared, and some people think they're taking a logical expedient action to get "back to normal." Both of them are wrong in their logic because statistically speaking it makes no sense to be afraid of covid. It also makes no sense to believe that a carrot on a stick to get "back to normal" will be the end of what's asked of us, even if there are in fact no serious prevalent side effects.