Just the presently known base morphs can make a lot more than 3700 combinations.

IMO, there are so many base morphs partly because the snakes spend a lot of time in mammal burrows lurking for prey to step into their parlor. There is a better chance for such snakes to survive because they are not wandering around for predators to see them easily. Even so, something like piebald is very, very rare in the wild.