Markets set efficient prices through competition. When I have just one vendor to buy from and have to pay whatever they demand or go without, it's recognized as

Markets set efficient prices through competition. When I have just one vendor to buy from and have to pay whatever they demand or go without, it's recognized as an abusive monopoly and market failure. Why is it suddenly okay for a bunch of vendors (of labor) to openly form a cartel, declare all competing vendors off limits through intimidation and violence, and extort whatever they can? Why don't unions have to compete for contracts and thereby set sane terms?