I wonder if the reason people are unhappy is that not winning and, sometimes, performing below your personal best is a pretty essential part of competitive sport. In order for someone to win, other people have to not win, after all.
I’m not much of a sports fan and i know nothing about gymnastics so it’s well outside of my area of expertise but if athletes withdrew from any event they felt they were not able to win on the day, then they would also deprive other (mostly younger and less experienced) athletes of their opportunity to win and enjoy the feeling of triumph and success that comes from that.
It raises some interesting questions in my view because it’s likely that, due to the nature of professional sport and the way athletes prepare for competition, not winning will always be damaging to their mental health. I don’t think any of that is healthy but it is the nature of competitive sport.