"So I guess my question is: why do you need generics when you have interfaces? These other (admittedly dynamically typed) languages make do without."
Going backwards, as you allude to, dynamic languages fulfill the use cases for generics, as long as you don't care about type safety, which is a thing that is true for the whole language anyhow so it's not much to give up.
For Go, the main problem is that when you're trying to be mathematical, with interfaces you get the worst of both the static and the dynamic worlds. You might like to define an interface that lets you add two vectors, right?
type Vector interface {
Components() []float64
}
type Add interface {
Add(Vector) Vector
}
which might let you implement an Add method on something that is a Vector as well, but you don't get a satisfactory result from either perspective. From the static perspective you can not, using interfaces, guarantee that someone doesn't add a Vector3 to a Vector2, meaning you must either panic at run time or have Add potentially return an error (that will generally not be necessary to check if used correctly, which is not a pleasant error to work with). From the dynamic perspective, you have to remember that what comes out the other end of that operation is
always an Add interface value, not a concrete type, so if you have a Vector2 and .Add(Vector2) to it, you don't get a concrete Vector2, you get a value of type "interface Add", which you have to manually cast back to a Vector2 if you want to do anything more than just keep adding to it.
You can make Vector2 have a distinct .Add(Vector2) method which does return a Vector2, but then if you also have a "func (v Vector3) Add(Vector3) Vector3" function, there is no way to declare an interface that both of those methods can meet, so you can not write any dimensionally-oblivious code that uses generic vector adding.
In "normal software engineering", Go's interface limitations are often not so bad, certainly not as bad as is often portrayed on HN. However, when you try to create a strongly-type numeric system (and you want it to be strongly-typed because that's also how you get good performance), Go's interface mechanism is basically worthless.