Partly, because Go doesn’t have generics (so you would need one set-type for every type, or fall back on reflection, which is rather inefficient).
Partly, because if all you need is “add/remove individual elements to a set” and “relatively space-efficient”, you can get a fair bit of that simply by using a map[yourtype]bool (and set the value to true for any element in the set) or, for more space efficiency, you can use an empty struct as the value and use _, present = the_setoid[key] to check for presence.