Accept the content would be a set list of items already in the game. But then that is just what I have in mind, there are all kinds of situations
Its not very easy to go those routes for a profit model based on dynamic interactions such as player transactions. Digital money has a lot of advantages in terms of how you can use it quickly. Some cryptos like Monero are nearly instant transactions of wealth and can happen without any need for infrastructure that dealing with banks would need.
One solid system written and updated on an engine like UE4 to support various coins would be far easier to setup for indie developers. In addition to that most coins are valued very low right now outside of bitcoin and will likely increase in value within the next 5 years on a dramatic scale. SO even if the percentages are low today those profits could sky rocket in the coming years.