when reactants and products are pure solids or liquids ,their concentrations are taken as constants , why ?

They are not taken as constants. But for pure solids the activity is taken as one. For liquids and gases, it is not one. The activity of solids is one because the active surface of solid which can take part in the reaction is almost negligible when compared to the liquid and gases. But what about for finely divided solids? Is the active surface area still negligible when compared to that of liquids and solids?

Chemical equilibrium problems are often "rounded off" for first year chemistry classes in the name of simplicity. Often, activity coefficients are neglected altogether. This is a reasonable approximation in an aqueous solution of simple salts that is not highly concentrated.

When dealing with solids, you usually dont talk about the concentration. Concentration is reserved for solutions or liquid mixtures. You usually use density for solids instead