Evander Holyfield and Roy Jones are P4P #1 and #2 for fighters who don't know when to give it up, respectively.
The probblem is that often boxing is all they know. Most boxers start off at a very young age. take boxer X. (not a real life boxer. Just an exmple). Boxer X starts going to the gym at around 10 years old. By the time he is 15 he is a ratted jounior boxer. By the time he is 18 he is one of the top amatuer boxers. Possibily an Olympian or World or national champion. Shortly after that he turns pro. As a pro he domantaes his compition. Either with quick knock outs or shout out decisions. He wins championship after championship. this goes on for about 10 years sometimes up to 15 and in rare cases 20 years. One night he gets into the ring and he gets beat or the fight is close. Now consider that Boxer X has all those years of being the the best. In his mind that fight was a fluke. The two boxers rematch and he loses or it is close again. Boxer X still is not used to this. All the years of being so great has inflated his ego. Because he is no longer a champion he has to take on a non contender or two to get back into the mix. He beats with ease the one or two non contendrs and gets another shot. He loses again. He goes back to fighting non contenders till he gets another championship fight and loses. The cycle keeps playing out like this for a while. As long as people will pay to see it. What it comes down to is that Boxing is all some of these guys know. It is hard to give up something that you have been doing since your were a young boy and that you used to be very good at.
the main reason is a lot of people who make their name in boxing come from tough backgrounds and don't have much to fall back on..it's really all they know and are good at. they wouldn't know what to do with themselves if they left because violence, abuse, poor education and poverty is a big part of what they knew before boxing came into their life. that's why fighters in general keep going past their expiry date.