Why is self only a convention and not a real Python keyword?

Because self is just a parameter to a function, like any other parameter. For example, the following call:

a = A()
a.x()

essentially gets converted to:

a = A()
A.x(a)

Not making self a reserved word has had the fortunate result as well that for class methods, you can rename the first parameter to something else (normally cls). And of course for static methods, the first parameter has no relationship to the instance it is called on e.g.:

class A:
    def method(self):
        pass

    @classmethod
    def class_method(cls):
        pass

    @staticmethod
    def static_method():
        pass

class B(A):
    pass

b = B()
b.method()        # self is b
b.class_method()  # cls is B
b.static_method() # no parameter passed