Why class containing extension method needs to be static ?

As we understand that when we create extension method on any type, we create it

as static method. But, class containing this extension method is also needs to be

static, otherwise we get compile time error. I didn't get proper answer for this

why this class should be static in this case?

Only for accessing static method, we don't need static class. That we can de even

if class is non-static i.e. we can access that static method by

ClassName.MethodName(). But, why this class is to be static in this case?

Can someone please answer this?