Since you cannot create an instance of a static type, the behavior of static struct would be exactly the same as the behavior of static class. So, there is no reason for creating them. I think it would be theoretically possible to have a static struct but it would be confusing – how would you choose between static class and static struct if the behavior of the two was exactly the same?
Note that static methods inside a struct are quite useful as you can use them for operations related to the struct, for example DateTime.TryParse etc.
Technically speaking I don’t think that the current C# compiler & runtime could produce something like a static struct, because internally (at the IL level) static class is a class that is marked as abstract and sealed. And I suppose that you cannot create a struct that would be abstract and sealed (in the IL).