Even ragtag rebel armies generally have a two-rank system of soldiers and "commanders."

Their recruits with any experience in other, "real" armies naturally try to emulate them. The desperate situation of a new rebel army probably doesn't encourage experimentation with new ways to organize.

If a rebel army has no-one with actual military experience, then it probably doesn't survive long enough to become a data point - or it just looks like a huge mob or a set of independent bands.