Well, look at it like this. Would you do the job you should be paid for the good of your fellow employees when you can hurt yourself and end any future income from it? At the end of the day, your teammates are not your brothers, they come 2nd. You and your family come first, then your team and teammates come second.
I think people forget, these kids are legit doing an actual job. These ones big enough to sit out aren't at the university doing this as a side activity. This is THEIR main thing/major.
Let's think of it a different way. Let's say you're a college student and you're going to get an A in a class, but there is an optional pop quiz team assignment. It's not mandatory you take this quiz as it does nothing to get you past A you already have and if you don't pass it you could potentially FAIL the whole class. Why would you even take this team assignment test with so much risk and no real reward? You wouldn't and this example is light because the only outcome is a grade. For these guys, a bad outcome could be your whole career ending.