Because if you have low number of slices (letās say 8 or 16) then using a random LFO to play from specific points in a random order is very difficult, since itās impossible to predict from where exactly it will play. With slices you can accurately set the start points and then the LFO can jump around between them.
Since I have rather short loops I will never have 128 slices, usually itās 16 or 8. Keeps things easier to follow. So there is some wiggle room for non-equal divisions, but you need to do some math.