system
A chat on subreddit /r/NoStupidQuestions
human
I guess there are two main reasons. 

1) the people that actually believe. 

2) social reasons - family, fear , lack of outside friend group, the specific reasons vary from person to person. but mostly fall into these imo
gpt
In my area (a deep south state) churches provide a huge amount of social support. I live in the suburbs and (I'm not exaggerating) there are 4 different churches on my outgoing street.

You meet friends in church. You find people to date through church. Job connections thru church. Cheap reliable day care thru church. Church often has better social services options if you are going thru a rough patch (like help with food, rent assistance, etc) than what you will find thru our local government.