There are lots of reasons I believe as I believe.
1. I was raised by two parents who went to church on Sunday and taught me to believe in God.
That one reason alone, indoctrination, is sufficient in answering the question.
There is little other reason to wonder why Christians cannot understand why anyone else is not Christian when indoctrination from early childhood instills into the child the invisible and undetectable are ultimate powers in the universe.
And one need not separate Christianity from this form of child abuse. Most other cults are pretty much the same.