Lancers aren't as good as you might think. despite their enormous charge and great attack stats, i've seen militia defeat Sipahi's, and light infantry without stakes beat Winged Hussars!
only if the player doensn't pay attention - cavalry that gets bogged down will get slaughtered by the greater infantry numbers, larger target they present and (if playing on harder difficulties) the stat bonuses the AI gets... however, if you micro-manage your cavalry, they can still destroy armies