Who's going to be the one to "watch over" the capitalist system? The government? Saying that capitalist corporations are vulnerable to corruption and hunger for power while governments are not is not only hypocritical, but completely false. Just look at history. Some capitalists have certainly done bad things, but governments have done far worse. Every genocide in history was perpretrated by governments, not corporations. Capitalism is not perfect, but it's the best system that's available.
Pure free-market capitalism is like leaving the henhouse unguarded. Having the government "watch over" markets is like having the fox "watch over" the henhouse.