That's not true. You can call an instance method from anywhere given an object. There's just no implicit 'this' in a static method.
Once you go functional, instance methods start to look like a special klugery for currying the first argument to a function. Go and Nim (for instance) hint at this more than a little, as well, with their OOP syntax.
Bundling two kinds of methods within a "class" starts to feel weird when you start using individual functions.