It’s the same reason people threw down some hate on Witten when he announced his retirement. Some fans don’t look at professional athletes as real people with emotions and problems just like us.
Some fans have strange, mythical expectations. Players should never have opinions that differ, they should only do what the fans want them to do, they should play despite any health risks, and when the player’s usefulness to the team is over, too many fans care nothing about the player when they’re gone.
I guess that’s just the way most people look at all celebrities. They never realize no matter how famous, wealthy or successful someone is, they are all still basically just like us but they have fame, fortune and celebrity.