Guys fight through injuries and get back on the field for many different reasons. They feel dedicated to the team to do so. They really really want to contribute and win. They want to improve their bargaining power next offseason, maybe because they're an average player. They feel they need to get back out there for fear of losing their starting spot, or of getting cut, or of getting cut the following training camp. Lots of reasons.
So to use the others like Fred, CJ, Urbik, and others to compare...well, it doesn't always apply.