Because to be super secure, not only should they not know what websites you visit, but also they should not know who their customers are in the first place.

Credit cards and other traditional payment systems tell the recipient who you are, they also tell the bank or financial institution that you are using a VPN, adding a third party into the private relationship.

Using crypto, a VPN provider can offer to provide VPN to anyone without collecting any personal information.

The VPN provider will still have exposure to all of their customer's IP addresses, but maybe they do stuff to mystify that too.