I assume what you mean is cryptocurrencies built on top of the blockchain technology.
The answer to your question is individuals. As simple as that.
This widely known across the crypto community article is a must-read for anyone interested in the matter. It was written by Jeffrey A. Tucker, the Editorial Director at the American Institute for Economic Research and a former Director of Content for the Foundation for Economic Education, back in 2014.
As former Federal Reserve Chairman Alan Greenspan said, "bitcoin (any cryptocurrency would fit) is really a fascinating example of how human beings create value or estimate or judge value. So long as people believe, they can sell it to somebody else or unload it on somebody else, and that's enough to create a market."