As coldknight said. If you don't have the power to do what you will, then where's the fun? RTW's replay value would be basically 0 if you could only do historical things. So instead the player has the ability to do as they wish.

why no create-a-unit or create-a-faction?
because there isn't. making those sort of tools would have taken CA much longer todo (if even possible) and rtw might have never been released until 05.