because its shit and if you are just learning its not good to rely on it as heavly as most of the people on zbrush central or /3/
it make bad topology, wastes geo, creates spirals, dosnt hold details very well unless you make it way too dense.>just do your topo by hand.
zremesher is fine for things like rocks or some hard surface stuff, something that isnt a character or hero objects... SOMETIMES
sometimes it works but most of the time it dosnt , its better to learn how to do retopology by hand because what do you do if zremesher dosnt work, or your job dosnt have zbrush. what are you going to do at that point>doing topo by hand will give you a better result every time if you know what your doing