There’s a very simple answer in pro football, that doesn’t enter the picture in other sports: in SOME sports, a player can go through the motions in meaningless games, with no harm done. I mean, now that the Toronto Blue Jays are out of the playoff hunt, their players COULD just “punch the clock.” They COULD just show up the last week, take a few half-hearted cuts and go home until Spring Training.
But if you’re, say, an offensive lineman for the Detroit Lions, you and/or the guys you’re supposed to be protecting will get KILLED if you try to give a half-hearted effort in the last few games of the season! You HAVE to give it your all, or you risk getting badly injured by guys who do.