Just out of curiosity, If you asked an evolutionary biologist why mosquitos exist, would you accept the answer, ‘because nature is stupid’?
If you want to think like a scientist, when confronted with some widespread property of a biological or social system, your initial thinking should be that you really don’t know until you study it, but given that it seems to persist in an evolutionary system, the default assumption should be that it brings some benefit to the system, or at least it did in the past.
Of the possibilities for why organized religion has persisted in human cultures since the dawn of recorded history, ‘people were just brainwashed’ is probaby the least likely to have any explanatory power.
I already gave a detailed description in this thread of a possible reason why religion persists: It’s a form of government for people at a community level. It helps facilitate cooperation within the group and allows for collective behavior without bureaucracy by building trust and improving communications. The church is a place where everyone comes together once a week, which helps solidify the community and act as a central clearing house for information. With a priest as authority, people can trust that their fellow church goers share the same values, and can be trusted to cooperate without the need for lawyers and contracts.
If you go to a small town, you may find that the church is not where you go to pray, but it’s also the local day care, the bulletin board has lots of non-religious community information, charitable services organize there, social functions happen on weekday evenings, etc. The level of trust fellow churchgoers develop for each other allows for a lot of ad-hoc coordination, sharing, etc. This even more true for Islam, where Mosques are often the center of religious and secular community activity.
I’m not religious at all, but I can look at religion without bias, and it’s not hard to find many ways in which it can add value to a community. And given its ubiquity in human history, it would be foolish to assume that it provides no benefit to society.