Why dont fighters fight more often?!

this has been bugging me alot lately, and i am getting real tired of it. why do the biggest names or prime athletes that can really do something wait so friggin long between fights? either it is a veteran that still has it or a new up and comer or simply someone who is still proving himself. i think a lot of these guys are wasting valuable time... i have a few examples: Mosley: not since January 09. (not counting May1) Marquez: not since September 09. (wont fight til July) Bradley: not since December 09, (wont fight til July) Mayweather: not since September 09. (not counting September) Dawson: not since November 09 (wont fight til August.) Hopkins: i know he is old, but one fight in 09? against a nobody?) there are probably A LOT more. more acceptable, but not quite good enough you have the likes of Amir Kahn and Paul Williams that both fought in December and are scheduled to fight in May. but i still think this is too much time between. one should think they would try to fight as often as they could while they still "got it". the scary thing is that 40 year old guys are more active: Vitali Klitschko: fought 3 times in 09 Roy Jones Jr: fought 3 times in 09 so what do you think? can you tell me why it is this way? back in the day it wasnt unusual to have close to 100 fights. now people retire with 35-45 fights. do you agree with me?