Because for loops are special forms to the compiler and don't require general parametric polymorphism to be implemented in the language. Parametric polymorphism

Because for loops are special forms to the compiler and don't require general parametric polymorphism to be implemented in the language. Parametric polymorphism adds a lot of complexity to the language, so I guess the go team has decided to just not have it in the language. I'm not sure I agree with that decision, but it is true that parametric polymorphism isn't all that useful for the types of programs go is usually used for. You don't want to use this implementation because it uses reflection, which makes it slow and means the compiler can't catch your errors.