Because they are proprietary, if they became quite popular, they would very likely be sold to either a competitor to shut it down, or enshittified by adding transaction fees.

As an older example, Paperback Swap was an incredible website where people could give away books in exchange for a credit, which could exchanged with another use for their book. There were no fees taken by the website, the only cost was shipping the books via media mail (which was extremely affordable, as the books would be wrapped in printer paper).

Paperback Swap was, at the time, a very popular website, with over a million books available. I used the service for many years happily exchanging books.

Then it was purchased by Amazon, which saw Paperback Swap as a threat to their used book market. They immediately enshittified the app, adding transaction fees and requiring you to purchase exchange credits or ongoing subscriptions. Within the few years the community was effectively killed, and now lives on as a shell of its former self.

Free, Libre open-source apps are the only long-term refuge from the constant churn of enshittification, as the community can always fork it to prevent bad-actors and venture capital from corrupting it.