One valid use of government power is punishing people who murder, and I'm not exactly sure what power cartels have outside of that.

I googled it and the Wikipedia page said they're inherently unstable, but I don't know how reliable that is.

In any case, I don't see how my second example isn't a cartel itself. All the bread companies are colluding to set the price of bread artificially high. The problem is there isn't much to stop new competitors (or to stop members defecting).