Why isn't Michael Phelps the greatest athlete of all time?

What defines the greatest athlete of all time? Recently I read a store on Yahoo and it got me thinking.

The premise of this story was that Michael Phelps was not the greatest of all time for one simple reason. He had never really faced an opponent that had a strategy against him. He never had to swim while someone was blocking him from the wall, never had to dribble the ball between defenders to win the gold, and never had to split the defense to dunk. 

While this is all true, I deeply disagree with the fact that an he is not the greatest athlete of all time. Sure, he didn't face the same challenges as Michael Jordan, Muhammad Ali, or Pelé, but that does not take away from his accomplishments. How can you compare someone who competes in basketball to swimming? or football to tennis? What counts is raw domination, and Michael Phelps has it.

Just like the names mentioned above, he wasn't bigger, taller, stronger, than everyone else. He took all the elements that his sport had to offer and personified them in a way no one has ever seen before. Sure, you can argue that he isnt the greatest athlete on the basis that someone else dominated their sport more thoroughly than him. However, you cannot discount his accomplishments as an athlete based solely on the face that his sport does not include the same facets that others do. He did won more olympic medals than anyone in history, regardless of sport, regardless of time, that is an amazing accomplishment.

He won multiple gold medals in a day, more medals than most countries have, and has more world records to his name than people have trophies from their childhood athletics. It is impossible to discredit is accomplishments based solely on the fact he isn't playing another sport.