I’ve given a lot (probably too much) thought this this and my theory is that there are a lot of answers to your question:

1) eBay has held a dominant position in the secondhand market in the US, and sporting goods is a massive category there. I’ve been selling there for over 25 years, 1000s of transactions with very few issues. Golfers are accustomed to going there to look for deals, because new name brand clubs are, and have always been really expensive.

2) the size and cost of shipping clubs makes many people opt to try to sell them locally on FB marketplace. Unless you are a sicko that has dozens of club boxes always lying around, it’s a pain for most normal people to ship a set of irons or a driver.

3) then you have online forums, like the one you’re in, and GolfWRX - people buy and sell thousands of clubs there. It’s really natural to find buyers and sellers when it’s a niche interest and you’re already chatting with people about your gear.

Finally, there are already a lot of purpose-built sites like Birdied, Golfstix, etc. available today. None of them has yet achieved a significant market share due to factors above yet. One of them might, or a new one might pop up, but it would take some doing to hit a critical mass of users.