The amount of money these elite level fighters make, and the extent of the promotion are part of what dictates such a sparse schedule. However, the bigger reason they don't fight more than twice a year is because it would put them in a much higher tax bracket.
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The problem here is that most promoters are either too greedy or too inexperienced in tax laws. They either cannot or will not protect their fighters from tax laws, which could cripple a fighter, or any athlete financially.
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I'll never understand how a lawyer, like Bob Arum, could ever allow Pacquiao to ever have tax problems when he "takes" much more than Pacquiao will ever make, yet never has tax problems himself. It has nothing to do with the fact that Pacquiao feeds and supports an army.
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Senators and Corporate heads are able to avoid such tax pit-falls. But athletes are prime targets and are unable to avoid the same.
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In plain English, If Pacquiao or Mayweather fight more than twice per year, the third fight might as well be for free.