Because there aren't any scientific ones.
There are plenty of creation myths that could be taught. But this shouldn't be the biology curriculum.
I think myth is trying to explain how the world came into existence without some supernatural element. No matter what theory you look at, eventually you get back to something that can't be explained without creation.
The big bang theory starts with the notion of a cosmic explosion that caused matter to be hurled in all directions. So then, under this theory, the question becomes how did the original matter come into existence?
Why do you assume creation is a myth?