A lot of people are religious/affiliated with a specific religion due to inertia. Church attendance was compulsory for a lot of history. There are not a whole lot of people converting to catholicism/protestantism/judaism from other religions. I only very recently found out the technical differences between lutheranism and methodism, largely details around how/why baptism and taking sacrament from what I can tell, but growing up, we always went to one church and not the other even though nobody could tell me why.

>If those people don't believe in the whole thing, why are they members in a church about it?

Usually they inherit the religion from their parents. They grow up with the church as family so even if they don't believe in the tenets/beliefs they still participate.