You do realize that the same thing happens to Mayweather right? Only difference is that unlike Pac Money May has never lost.
It's pretty easy to go undefeated when you don't fight anybody that poses a threat. I like Mayweather, I've pulled for him in all of his fights, but I've also said that he won't be remembered as a great fighter until he beats the great fighters that are still waiting to face him. Sugar Shane and Pacquiao are just two of several who would take a fight with Mayweather without question and definitely pose a threat. If you want the full list, listen to that interview with ra the rugged man, he names them all.
What Mayweather wants is a huge payday on top of a guaranteed win, so he can collect the cash and walk out of the ring without a scratch.
The point that Pacquiao has lost and Mayweather hasn't is moot. Look at who Pacquiao has fought compared to Mayweather and then see if Mayweather being undefeated really means anything. In Pacquiao's entire career as a fighter, he has 3 losses, two of which were 10 and 13 years ago. Mayweather can try to make Manny look like less of a boxer for being knocked out twice, but A) He lost by KO the first time 13 years ago and the second time 10 years ago and B) Pacquiao takes fights where it's possible for him to get KO'd.