What happens if you increase enemy AI? Do you think they'll magically be able to live longer?
uhhh.... yes? Imagine having every enemy in a game be another human player (one that isn't awful) and try telling me they wouldn't provide more of a challenge. People do stupid shit sometimes but bots do stupid shit most of the time.
Here's a simple scenario. What if an enemy in a game sees the player before the player sees him? What would a person do?
Attempt to snipe? Set up an ambush/trap? Sneak up on him?
Those are all sensible things a person might think to do. Bots never do any of them. Bots just notice the player, most likely shout something, and start firing haphazardly. Replace any enemy in any shooter with a cardboard cutout and the experience wouldn't change a bit.