Why groups don't defend as a whole?

[BladeOfSharpness]

NPC caravan travels, stumble on a group of hostile and ... the wounded are left dying on the ground, everybody fire without any consideration for friendly fire. The rear guard can be ambushed, the rest continue, etc.

It just don't make sense.

[Spocklw]

Because there is no AI, just a set of basic algorithms. The whole game is balanced around pawns being "dumb as a bricks", that's why  you are able to defend raid with many times more raiders than your colonists.

[Kirby23590]

It's not easy coding combat A.I for a game like Rimworld, though i love challenging A.I, though programing them to save their wounded might make sense when the Enemy is fleeing at a cost of their lives for sacrificing to save their comrades live, if it's easy or hard to code it...

But it would also slow down your computer, especially if there is a ton of Raiders and it would cause and make the game lag as well...

[ashaffee]

That would be such a huge over hall to everything balance in the game I would cry for Tyran. The system works pretty well right now. Changing it would cause a cascade of work that would never end. Plus the reasons mentioned above.