I've never heard this question before, but found it interesting because I have talked to vegans who think eating vegetables gives you enough calcium.
Myth 2: Spinach is as good a source of calcium as milk. FACT: There is more calcium in 1 cup of milk than there is in 16 cups of spinach. One will need to eat more than 48 cups of spinach to get the recommended daily intake of calcium (USDA, 2010). Furthermore, milk contains Vitamin D which enhances calcium absorption (Wasserman, R.H. 2004. Vitamin D and the dual process of intestinal calcium absorption. Journal of Nutrition 134: 3137-3139.