无法在静态方法中使用实例变量

为什么不能在static method中使用instance variable ?我知道静态方法调用没有创建类的实例，但什么限制非静态变量在静态方法中使用?

class MyClass
{
    // non-static instance member variable
    private int a;
    //static member variable
    private static int b;
    //static method
    public static void DoSomething()
    {
        //this will result in compilation error as “a” has no memory
        a = a + 1;
        //this works fine since “b” is static
        b = b + 1;
    }
}