For almost all demand curves, the resulting marginal revenue curve is often to the left of and steeper than the demand curve. The marginal cost curve has its distinctive U-shape, and a particular portion of the marginal cost curve is the supply curve. So the result can be shown below:
The figure above indicates that in almost any case, because MR is to the left of D, if a firm produces the quantity at market equilibrium, MR < MC, which means that the firm would be actively losing profits. So, why do firms even bother going to market equilibrium - from a theoretical standpoint?