Why interface cannot have static methods in C#

Because an interface is a "contract" or an agreement between the consumer (caller) and the provider (callee). An interface describes what and how the calle will provide functionality. There is no need for static members provided by a third party. Static members cannot be overridden by a provider so they do not belong in an interface.

.NET Questions (CLOSED) - Why can't you have static methods in an Interface?[^]

Another way of looking at it:

If an interface could have a static method:

interface IFoo
{
    static void Bar();
}

class Foo1 : IFoo
{
    public static void Bar() { ... }
}

class Foo2 : IFoo
{
    public static void Bar() { ... }
}

and you invoked it:

IFoo.Bar();

there would be no way to known which implementation you would end up calling. Would that call Foo1.Bar or Foo2.Bar?

You might decide you want it to call both implementations. Which might work, until you introduce return values, or ref / out parameters:

interface IFoo
{
    static int Bar();
}

class Foo1 : IFoo
{
    public static int Bar() { return 42; }
}

class Foo2 : IFoo
{
    public static int Bar() { return 84; }
}

int x = IFoo.Bar(); // What is the value of x?

An interface is a contract for things you can do with an instance of a class. A static interface, or static members on an interface, doesn't really make any logical sense.

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Edit: Since this has been dragged back up into the "active" list, it's probably worth saying that as of C#11, this information no longer applies. Interfaces can now have static virtual members:
Explore static virtual members in interfaces | Microsoft Learn[^]