If monopolies are bad, why then does the government give certain firms monopoly power?

Although monopoly may be considered bad, the government may give a monopoly to firms for various reasons. For instance, the government may grant a company a monopoly to manufacture critical products in the economy, i.e., armaments. Armories can have a devastating effect when they land to the wrong hands; hence the government may want to regulate how they circulate in the economy. Secondly, it may give a monopoly to keep the cost of products relatively lower, especially for the essential goods. The lower cost results due to economies of scale where purchasing raw materials in large quantities result in higher discounts that translate to lower production cost.