The kinds of parametric polymorphism implementations mentioned here do not represent the full set of options. Charitably this represents an ignorance on the part of the Go developers, but really it has felt like an apathy towards the field of programming language design and research. Intensional polymorphism (basically passing the type of a parametric argument as a parameter, and either using a specialized implementation of the function or one that operates over a boxed value) has been known about for years and is used in languages like Haskell: https://www.cs.cmu.edu/~rwh/papers/intensional/popl95.pdf