Never used Go, but I've written a lot of C in the past and I think the point you're missing is that they probably intend for you to use a special purpose data structure when a map or slice doesn't fit your needs. When it's special purpose, there's no need to use generics.

When I needed a generic data structure in C, I would do code generation (but only after I'd have a use for it in more than one place). The number of times I ever needed to do this for anything other than an array or hashtable is (not really surprisingly) vanishingly small.

So, coming from the perspective of Go is a better C (that avoids the mistakes of C++), this choice makes sense to me. I don't necessarily think it's the right choice -- I think generally generics are useful, but tend to use generic functions in my C++ code more than generic types or containers.

Might be worth noting that I work in game development, where using a library data structure isn't really considered for most things, because a special purpose one is likely able to be higher performance, due to assumptions it's able to make, operations it doesn't need to implement, etc. I guess I can see the situation where not being able to find an off the shelf generic data structure is frustrating, but I think it might be a bit overblown.