The first classes I tried to go for every semester in college were religious studies. But they were almost always the first filled. Seriously. Who doesn't want to learn a whole semester about Christianity through the Middle Ages, the roots of Islam and the rise of the Islamic Empires, the earliest roots of Judaism, the Crusades, Renaissance-era theology, all the great schisms and councils, conclaves, scandals, heresies, religious wars NOT fought against Islam....I got a lot of that in my history classes anyways so there was plenty of overlap, but I found all of it fascinating. I got some of it to a degree in my highschool AP classes, too. But college is where I really dug in. I still have every single textbook I bought for those classes.
In American public education though, it's not just people don't want their kids being "indoctrinated" (which is code for "I don't want someone presenting alternative religious systems in any details so my kid can think for themselves.) I think it's also that some people don't want their religions judged in a qualitative fashion. It's an easy subject to misrepresent, regardless of who is teaching it or which religion you're talking about. I remember plenty of misunderstandings and debates even in college where the teacher had to stress "look, I'm not making a value judgment here." It's hard to even teach the facts of religion in America without making someone hostile.
Religion's status as a charged topic goes all the way back to our founding. The separation of church and state is written straight into the American Constitution.
Consider it's a nation founded by people seeking religious freedom and freedom from religious persecution. There was extremism even in the earliest pilgrim communities, the kind you get when you have a people free to believe what they want with no one telling them what to do, almost belligerent in its freedom of expression. America has carried that same sentiment forward through its history. There's long been a sort of defensiveness in American Christianity, even though its the dominant religion in the land. When they took the 10 Commandments down in front of that court house, or when atheists try to get "Under God" taken out of the Pledge of Allegiance, Christians go right back to the pilgrim's desire to not be oppressed.
So you see there's actually two ways to interpret the separation of Church and State. One is that "There's no religion in the State." Ie. public schools can't teach religion. The other is that "The State can't touch religion because that's oppression." Ie. the State can't make you vaccinate your kids because it's against your beliefs. Taking Under God out of the Pledge of Allegiance actually utilizes both trains of thought, because one side views it as a State endorsement of Christianity, while the other views it as the State trying to oppress Christians by taking it out. Both viewpoints are correct. And so some kids die because their parents don't believe in vaccines, and children aren't allowed to hear about religion from the people who, arguably, should be able to give them the most objective view of it.
Like most political things in America, it's a mish mash of contradictions, personalized interpretations and sacred cows. Which is why the solution we've come to is "Parents teach religion in the home, public schools touch it with 10 foot poll and if you really want to learn about religion, you go to a private school (many of which are run by religious organizations) or you go to college."
Which still doesn't free us of the debate because of school vouchers (tax dollars for kids to go to private, often religious, schools.) We're right back at the Separation of Church and State. Americans are literally incapable of letting go of the religious topic. And that's not even getting into the anti-Semitism, Radical Islam or general bigotry.
And lastly, I have to give the disclaimer that there are plenty of moderately religious people who happily get along with their religion and others in the modern era. Though they speak and their words are recorded, for some reason they're always drowned out by either sides.