Chamberlain, Jim Brown, and Babe Ruth give us little frame of reference for evaluating their greatness. In the case of Brown and Chamberlain, they were so superior athletically to the competition, it was a case of men among boys. A player as dominant as Wilt should have won more titles.
In the case of MJ, he played in an era when there were many (almost) equally gifted athletes, and he still dominated. For example, Vince Carter could do stuff that MJ couldn't even dream about. Yet, Vince never even played in a single Finals. MJ managed to win going against other great players.
The argument can't be settled deinitively, but it's this debate:
a) The Bulls were so great that they never lost. OR
b) We never got to see just how great they were because they never lost.
There is a strange way in which losing validates a champion. Seeing the Lakers come back after losing in '84, or the Cowboys come back after losing in '95, helps fans frame their accomplishments. They were great because they beat the greatest. It's why - although we appreciated Tiger's greatness - we wanted, just once, for somebody to bow up and take him down, to see how he would handle a legitimate rival.