Because having to play with the same company over and over again makes it interesting and allows for every fight to be about the same. This makes balance easier as its unlikely you will run into a situation where you will have to face the rock to your scissors.
At least give us a spot to put units that aren't currently in the company makeup so we can take advantage of defending games and attacking games more. That way you don't lose the XP/PP on the units, but they don't take up resources either. Allows some variation on a scheme at least.
Actually AMPM, there will be something like that coming in
A small reserve area.