Why don't Fighters Fight at their Natural Weight?

Wouldnt it be much healthier and easier instead of losing 30 to 20 pounds? I know they say they do it for a size and power advantage at the lower weight classes but still that has to be unhealthy on the liver and kidneys.But do you think that is the main reason they fight at lower weights? These are the possible weights not hard facts but I heard from interviews and such how much they weigh and estimated their offseason weight from the hbo unofficial scale. manny pacquiao fights at 130-126 and walks around at 150 Erik morales fights at 130-135 walks around at 165 Ricky hatton fights at 140 walks around 175 and up Arturo gatti fights at 140 walks around at 165 170 miguel cotto fights at 140 walks around 160-165 joan guzman fights at 122-26 walks around 150 kermit cintron fights at 147 walks around at 175 antonio margarito fights at 147-154 walks around 170. Jermain taylor fights at 160 walks around 180 Fighters who walk around at their fighting weight or close to it are: Floyd mayweather fights at 140-147 and walks around like 145 Marco antonio barrera fights at 130 walks around 135 demarcus chop chop corley fights at 140 walks around 145 bernard hopkins fights at 160 walks around 168 shane mosley fights at 147 walks around at 155.