Because popularity is not a way of testing scientific accuracy. It's not even an argument, except for how popular something is. Most people believe a lot of false things, and they aren't taught taught anywhere except for social science classes that cover parts of how sometimes commonly held "knowledge" is not a fact.
People still believe in a flat Earth, but if it's even mentioned in a science book it is inevitable that it will be followed by saying it's false. People can believe what they want, but if it's not science it doesn't belong in a science class. Much like how history has no room in a math class. Creationism can be taught in classes where the context is appropriate, such as philosophy or comparative religion, but it doesn't belong in a science class.