It breaks my heart to see once great fighters continue fighting on when they are clearly old and no longer have it, constantly getting teed off on by some young and hungry kid on the come up. When I was a young boy, I was upset that Lennox retired instead of rematching Vitali, but as the years went by, I understood that it was the right decision. At age 37, you can't keep fighting forever.
Money, pride--think they have one more good fight left and want to end their career with a win (Bernard Hopkins--ouch!).
There is a mentality it takes to be a world class athlete. Guys like that don't have quit in them, they have supremely high self belief. That same self belief is what makes it hard for them to admit they no longer have what it takes.
Just about always its because of the money. Hardly any boxers are able to live off of what they made in the ring. Most end up broke and broken. Some are able to have some money but end up working a normal job like the rest of us.
I'd say money and due to not having anything else to do. It's a short lived career, and it's all they've known since a very young age. They miss the buzz, energy and excitement of the big fights.