The risk of a well-diversified portfolio to an investor is measured by the standard deviation of the

Question:

The risk of a well-diversified to an investor is measured by the standard deviation of the portfolio's returns. Why shouldn't the risk of an individual security be calculated in the same manner?
Portfolio
A portfolio is a grouping of financial assets such as stocks, bonds, commodities, currencies and cash equivalents, as well as their fund counterparts, including mutual, exchange-traded and closed funds. A portfolio can also consist of non-publicly...