Because they understand what Bandcamp is charging for?
The page for Bandcamp Pro lists all the feature you get (and everything that isn't listed there, is stuff you don't get): https://bandcamp.com/pro
Again, clearly advocating for the platform and missing the entire message.
A musician can easily self host a site that does exactly what bandcamp represents for them. Also, most serious artists are already distributed to platforms that do exactly what Bandcamp does and more.
There is very little value in bancamp, unless you are perhaps trying to sell songs with lots of uncleared samples in them.
But if you signed up for Bandcamp Pro for $20/month, then clearly you either were happy with what you were getting, or you misunderstood what you were about to get. But personally I think that pricing page is pretty clear about what you get.
So then what was the 20 dollar monthly fee truly for?