Because they are fighting no hopers 3-6 times a year. That means that they are the only FULL TIME BOXERS in the field of PART TIME BOXERS. Look at Duran's resume. He will fight a fighter with a record like 17 wins,12 losses, or with similar one three to 4 times, then have a megafight, and back to the same fodder for 5-8 fights. These bums hold day-time jobs as firemen, laborers, ect and train at night. Basically, those bum fights serves as to maintain conditioning before the next big fight.
Some boxers put punchers together properly some don't because they are in love with trying to imitate people like RJJ. Either way that didn't stop Morales from struggling and losnig to a crude slugger like Maidana or getting embarassed by Garcia, who I don't even rate highly. Pavlik and Berto could both compete in the 70s or 80s if you put them back in those eras as they were in their primes now, doesn't mean they would be as successful as they are now, but that has little to do with their trainign and more to do with less sanctioning body BS and greater fighting between people on the way up. But then again, I dont rate Berto highly now, especially after the robbery from Collazo.