The same reason anyone gets paid a lot of money; they can get it. By that I mean that if someone has a unique skill and enough people are willing to pay to see it displayed, that skill becomes very valuable. As a player realizes that, he will refuse to play until he gets enough money (which is negotiated through an agent) from the team that wants him. Sometimes, one team will compete with another to sign a great athlete which makes the price for his talents go even higher.
This works the same way for movie stars and other celebrities. In Economics, it's called the law of supply and demand. When the supply of a commodity (sports talent) goes down, and the demand (the desire for people to see it) goes up, the price goes up too.
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They have to spend more time than other jobs, traveling a lot and training, so that they don't spend much time with their family. In addition, athletes have the responsibility of their country's tradition and their careers are not long as other careers, because if they get injured they will lose everything. More than not they're certainly extremely gifted in what they do because there is a small percentage of people who make it to the pros.
Because their doing entertainment and people who do entertainment always earn a lot of money. That's just the way it is.
To do with fitness required, training levels and 'celebrtiy/fame'. With all due respect they are being paid to much and the Army deserves more.