@andrarchy already gave a pretty good answer.

It depends also on what kind of system it we are discussing about. If we assume that a certain blockchain is perfect and it doesn't need ever again any kind of improvements, then it doesn't necessarily need a governance system. But so far we are very far away from perfect blockchains. There is always something that can and should be improved.

For example, Bitcoin is in trouble with the blocksize debate. They can't decide what kind of actions should be taken to move forward. Somebody might say that this is a problem of non-existent governance, but another perspective is that there is governance structure, it just doesn't work very well. Participants in Bitcoin economy are still doing decisions all the time, about what not to do. They are deciding that all improvements that are suggested so far should not be implemented. So there are people who have power – they have a power to say no.

Bitcoin governance structure is a good example of ad hoc governance which just happened to arise. Nobody designed it.

If you want to know who are the government, you have to ask who have the power to say yes or no to any proposed change to the protocol.