Just pretend for a minute that you're not a player who is "looking out for what's best for him".
If a team sells tickets, and the best players quit as you approach the end of the season, how do you feel about that if you are a season ticket holder?
If you are a sports / broadcasting network, and the biggest 5 or 10 players in the sport, who you have promoted all season long, quit just as the playoffs are starting, how is that going to go over?
I just got a refund on my cable bill last fall because broadcast rights holders demanded money back for the lack of broadcast product from various sports leagues.
How would you have felt if Melo had decided in early March 2003 to sit out the rest of the season to avoid injury?
This is not an abstract issue. Leagues are going to lose fans if this becomes commonplace.
Loss of fans (or loss of eyeballs) means loss of revenue, first to the broadcaster, but then to the program or franchise, who loses broadcast rights fees.
We are only at the very beginning of this trend. Saying "quite a few" college football players did it is not really accurate. Alabama, Clemson, Ohio State, all those teams had almost all of their star players for the football playoffs. Trevor Lawrence didn't sit out at Clemson.