Static classes can't be instantiated in the first place, so even if you could declare non-static (instance) members, they can never be accessed. Since there really isn't a point allowing it for that reason, the language simply prohibits it.
Bear in mind that static classes are just the classes, while there are two things that are directly related to non-static classes: the classes themselves, and the instances/objects of the classes.
A non-static class can have both static and non-static members so that the static members apply to the class, whereas the non-static members apply to the instances of that class.
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