Why are patents sometimes referred to as monopolies?

Why are patents sometimes referred to as monopolies?
A monopoly is when one person or company has total control over the market for a certain product or service. Like in the Hasbro game of Monopoly: when you own all the properties of the same color, you have a monopoly.
If you patent a product, it means that your product is documented so no one can take your ideas and/or designs and claim them as their own. So you are the only one that can claim that product and you have complete control over that product's market. Therefore, you have a monopoly on your product.