I am not a nutritionist and can't explain all the WHYS, but cattle need the fiber in their diet.
Yes, you see the "whole corn" in their manure, but if you had the sophisticated equipment to dissect the manure with cracked/processed corn, you will find just as much corn. You just can't SEE it.
I take fall born weaned calves in April, start them on the WSC diet. Granted, these are Simmental steers, so they are already 550-600#/6 month old calves. I finish them in October with an average (minimum) of 750# hot carcass weight. Full hay and/or grass from day 1 to finish.