Milk does contain calcium... but they are also high in protein and diets that are high inprotein, especially animal protein, cause more calcium to be excreted. Meats also contain large quantities of phosphorus, which can impair calcium balance.

– Hegsted R B, Barden H S, Zemel M B, Linkswiller H M,
“Urinary calcium and calcium balance in young men as affected by level of protein and phosphorus intake,”
Journal of Nutrition, 111:553-62, 1981