Does a thin chainmail turn fatal wounds into minor ones? And does a car save you from a concrete slab falling from a height of a couple of floors?
A horde of horses crushes peasants and... encounters a wall of trained soldiers, shields and spears, nothing extra.

And again, there are other types of troops. Why send the most powerful part of your army to certain death? Which, moreover, has only one chance to attack, because the value of a Knight stuck in battle is much less.
However, the range of bow shooting was still better than other types of ancient weapons, and the accuracy at that time did not cause complaints, as shooting was usually done at a line of soldiers. In a real combat situation, an arrow almost never pierced any armor, at least in ancient times. However, it is difficult to completely protect the body from all sides with armor of sufficient strength, so massive shelling led to numerous wounds and loss of combat effectiveness of enemy troops.
Except the ones we've made