It's because God gave us a will. For humans, they have a will to follow God and also they have a will to deny him as their savior. Some people are doubters, they ask themselves if God is true, why he didn't show up when I have a problem and such. In that way, they lost their faith and switch to a religion that they think can save them. It's unfortunate that some people will just create a religion to control and manipulate people for their own doings. Because they are using the name of God, however, it's the karma that will get them revenge.