[foldr]

The compiler also has to generate the specialized code. But I suspect that the main factor here is just that the presence of generics in a language encourages the use of generic programming to an extent that wouldn't be practical in a language without generics. Take e.g. the boost geometry library (C++). Theoretically you could write a similarly generic library in Go using code generation, and it would probably take ages to build too. Of course, no-one would actually do this.