There's 2 opinion on that:
1. "They blend in because they don't want trouble." If you ever seen the Japanese School, they quite hate difference. Anyone who is sticking out like a nail will be hammered down. By the school, or by your pees. If you can't blend in with the others, you will be labeled as a delinquent. But if you can't even be part of the delinquent or be a part of something, you are an outcast. Either you are a normal student, a delinquent, or an exiles. no in-between. That's why they have this idea that they need to blend in with the normal student to avoid trouble of either being hated or being worshiped. It's not about how fun the trope is, but about how "logical and made sense" the trope is. (
This and
this should give you some understanding about what i'm talking about)
2. "They are a chuuni." The OP MC HAD to blend in with the crowd of normal is because that is the "cooler" theme. A very good example of this is "Eminence in The Shadow", they don't even hide the troupe by making the MC want to hide to avoid trouble. The idea that the MC is like a hidden boss, is fascinating for some people. It's like they are a crouching tiger waiting to show their magnificent when the time come. They like the idea of people being surprised by the fact the person they look down so much is actually above them and etc.