Several reasons. Some don't CARE that religious beliefs cannot be endorsed by the state. Some don't understand that creationism isn't science and has no place in a science classroom. Some just don't understand what the Constitution actually says, and are under the misapprehension that it just states the government can't mess with the church.
It is not a two way street. The constitution limits the power of the state, not the church. No law can regulate belief or political action by anyone, including the religious. People should have learned that stuff in the 7th grade. - Anyone who thinks that separation of church and state means that there are any limitations on the church needs to get a Junior High School text book and learn the very basics of constitutional law as practiced in this country that they obviously missed the first time.
The separation of Church and state was to keep the state out of the church not the other way around. History tells us that this country was founded on Godly principles and our forefathers did not want the state going into the church and having the ability to govern them. It wasn't to keep church out of school. In fact schools started in the local churches!!!