To those who say frameworks are needed because the stdlib is not enough: No, the stdlib defines very clear interfaces that libraries can implement. The result is that you can drag and drop middlewares from multiple packages into your codebase, because they all conform to the universal middleware signature `func(http.Handler) http.Handler`. Want CSRF? Want Sessions? Want Auth? They all exist as separate packages (check out the Gorilla toolkit), but the beauty is that they all work without needing to know the existence of each other.
Frameworks like Echo and Gin eschew the universal middleware signature `func (http.Handler) http.Handler` for their custom built ones, and as a result cannot tap into the ecosystem of middlewares that target Go's net/http. Between a stdlib package and a third party package, which do you think is more stable?
You are not constrained to net/http's router. Chi is a good example of a third party router that implements features not in net/http but still conforms to the `func(http.Handler) http.Handler` interface, and as a result middlewares that work for net/http will also work for Chi.