I haven't seen the movie yet, but horses tend to not run into a solid wall of shields and spears. Calvary charges at men on foot was meant to frighten the line and make them break formation and run. Chariots would strafe the front lines with missiles mostly, firing on approach and retreat. A horse has a sense of self-preservation, and will most likely pull up before a shield wall bristling with spears. Most Calvary used in ancient warfare would be used to contain the wings, or flanks, of formations, or to run down fleeing men. If they were mounted archers, they would be used in skirmishes and to harass the enemy. Ancient warfare could be somewhat boring until one side tried to flee instead of an orderly retreat, or was out flanked. Hollywood must make every scene dramatic. I hope this helps you. Not a historian, just an avid reader.

There was a review on /r/BadHistory discussing the movie.

http://www.reddit.com/r/badhistory/comments/2pawlw/exodus_gods_and_kings_or_how_i_learned_to_stop/