They care a lot about control over credit markets. The entire premise of central banks controlling inflation via the money supply becomes precarious when people are able to issue and access credit outside of the controlled money supply.

You're entirely right that they want their cut, but they also don't want to end up like a country such as Indonesia or Vietnam where all the financing has to be organised in the USA. The US FED can print money during a recession to ease credit conditions; many non-western countries have no such luxury because while they might have a sovereign currency, all the serious enterprise in their country is financed in New York and cities in Western Europe.