It's not really that complex.
Humans have historically been better competition than AI. Writing AI that is evenly matched with a human so as presenting a challenge that is tough but not unwinnable is much harder than just playing against another person.
> Maybe it makes joining the military not too unappealing for teenagers.
Someone should have told the US Army: https://en.wikipedia.org/wiki/America%27s_Army
(Surprisingly for a government project it was pretty playable)
> Writing AI that is evenly matched with a human so as presenting a challenge that is tough but not unwinnable is much harder than just playing against another person.
Also humans are uniquely... human.
I play one of those extraction shooters and even a much higher ranked player, who would normally have no issue downing my team of three in an open fight, will eventually get worn down if we hide around and harass them. Also they might just lose patience earlier and start making mistakes due to that.
Hard to model something like this because people are different and react in complex ways.