You get folks who just do it as a hobby (because it's not that expensive), die hard folks who live it (but aren't that wealthy), plus ultra-wealthy folks (who are generally jealous of the former!). I think it's the peace and tranquillity that attracts.
Add to that, that at least in the US, Trout Unlimited is without a doubt the most politically-connected pure sportsperson lobbying organization.
What you described is also true for skiing, scuba diving, mountain diving and any number of other outdoor pursuits.
Fly fishing feels more like bird-watching to me though: in that it's relatively easy and cheap to do, but not a thing you would ever do accidentally.
[1] https://outdoorindustry.org/wp-content/uploads/2015/03/2018-... [2] https://www.statista.com/statistics/376710/active-skiers-and... [3] https://www.dema.org/store/download.aspx?id=7811B097-8882-47... [4] https://www.businesswire.com/news/home/20170315005391/en/Cam... [5] https://www.statista.com/statistics/191240/participants-in-h...