Madonna has lasted way longer than anyone else, duh.
And they're compared because they're suppose to be the two most successful females ever (even though Celine is #2). Their music is completely different, they've never been direct rivals (like Britney/Christina, Selena/Demi, etc), and their peaks were two different decades so the comparisons make no sense, really.
Only pop stans compare them anyways. Nobody in real life would bring up their name in the same conversation.