Insurance was mentioned quite a bit. How does it work for you average 5k - marathon? I have never had to join USA Track & Field. How do those event work?
Now swimming is like triathlon. USA Swimming seems to be the governing body and you need to bbe a member to swim for the top clubs. I also understand it that in order to be a US Record it must be in one of their (or a FINA) events. So you can't set an age group record at your youth summer rec league swim program. I don't know how High School and NCAA swimming works.
But this isn't just Olympic sports. I had a neighbor who's 9 year old played baseball all year round on multiple team. For one of his "travel" teams he had to also pay a membership to a national organization. For basketball AAU seems to be the governing organization for youth once you get above rec leagues (outside of HS). Soccer probably has something similar as I understand it the better kids play on club teams rather than HS.