"Human beings cannot absorb minerals satisfactorily from inorganic chemicals." - Weston A. Price, from Nutrition and Physical Degeneration, page 294, lines 10-11.
Since it isn't absorbed well, maybe it just irritates things as it passes through, or else serves as a catalytic site for bad reactons to occur. Cancer has risen at the same time calcium supplementation has risen.
I know some really old people who never popped a calcium supplement a single time. Me, fewer than a dozen times in this life. Calcium supplementation makes little sense to me, since it is so widely available.