Because there seems to be a limit to how many they drop, even if you get looting rights. It seems to be 3 per general, 2 for captain, 1 per anything else, even if more people than that get looting rights. So it's quite tiresome when my beetle's spent 5 minutes soloing a general to 50%, only for a dozen guys to then jump in and spend the next 3 min helping to get him the rest of the way.
It's the same for the colored armors, actually. The guys with ringmail, chainmail, shields, or cloaks are quite uncommon, vs guys in platemail. I'm getting rather sick of guys who run in, hit just enough for looting rights at the end, then when the loot gets split out, they get to loot those colored armors since the RNG gave those to them in their loot, while I have to wait an extra minute or two for it to hit the 'common' loot (if they hadn't already snagged it). Just way too much plate, and way too little of the others. Even if # wearing plate is going to be higher than chain or ring, they could all at least be wearing a cloak and some sort of shoes.