Real Army Vs Army games?

[Hazzard]

A while ago in this section of the forum someone asked about games where you were in a proper army.
One person said there weren't many of them due to system limitations.
But Star Wars Battlefront 1 managed it, and that is nearly 8 years old and it managed it. So why is it so rare?

 

[Esotera]

Artificial intelligence is quite expensive (it takes up a lot of resources in your console/computer), so it's undesirable to have thousands of NPCs trying to kill each other/survive all at the same time.

But another point is that as a player, how many enemy units are you actually going to encounter in the course of a battle? It's going to be quite a low percentage, and as the game's focus is on the player, resources could be better spent making a small number of units look good than a large number of units look average.

Then again, some games manage to get round this, but it must be a major headache. And this is only my best guess.