Groups of units get a stack bonus that maxes at eight. This makes eight-man formations fairly common. Any special bonuses apply to the entire stack.
Stacks without a leader are forced to auto-attack when in contact with units from non-allied capitals. With a Commander the fight can be directed, or the group can choose not to initiate a fight.Erf-b1-p040a
It could also be that it was simply more efficient for the stacks to be separated, as their move stat was so different that, rather than slow down the fastest units to have them all arrive at the same time.
It is unknown why everyone doesn't just form massive superstacks to maximize bonuses; even if the stack bonus maxes at eight units, it seems that stack-range bonuses, such as the high stack-range component of Leadership, should be spread to as many units as possible. Also the loss of a single unit in a stack of eight would begin reducing the bonus while adding nine or more would provide a buffer.