DISCLAIMER: This is all just stuff I've heard over the internet, so I can't say whether or not any of it's true. I just think they're interesting bits of speculation that are worth looking into.
I've heard that because of the way most food today is prepared, it doesn't have as much nutrition as it used to. For example, broccoli grown today might be grown in less fertile soil, or more chemicals might be used in its production, thus reducing the vitamin/mineral content compared to what broccoli grown say, 70 years ago might have. Even people who eat healthy are bound to be slightly deficient in some nutrient or another, and so multivitamins are what they use to cover that deficiency.
I've heard that because of the way most food today is prepared, it doesn't have as much nutrition as it used to. For example, broccoli grown today might be grown in less fertile soil, or more chemicals might be used in its production, thus reducing the vitamin/mineral content compared to what broccoli grown say, 70 years ago might have. Even people who eat healthy are bound to be slightly deficient in some nutrient or another, and so multivitamins are what they use to cover that deficiency.