It’s called the concept NFT, that implies “limited” by design.
They are different ideas coming from a group of forum members or even Captain himself , eventually created as a field by Captain.
It’s started as few couple of years back and reached 500+ now…lol
You can read more here https://forum.enlightenedstates.com/t/official-nft-guidelines-discussion/58313
I think It’s the problem with Gumroad and PayPal. If they are excluded means you need to understand that Captain have tried all possible ways to include it but still wasn’t possible because of too much trouble ( losses too) from them.