Yeah, OKB Zero and the Soviet Base Camp immediately spring to mind as being infested with enemies. They're everywhere! (And I love it, lol)The main base does send out choppers and helicopters, but only at high active combat readiness. It also sends jeeps with soldiers from other bases. Not sure why you think this part is somehow unfinished? Also they will sometimes send a four man team outside the main base perimeter if they have the manpower. There's even a guy in charge who will be like "on me" and if they take awhile to get there he will mumble they're slacking. At that point they'll go pretty far afield and issue orders to split up. That said, them mostly sticking to bases and outposts is clearly by design. They wanted the zone in-between sandboxes to be a safe staging-zone. Get a combat alert? Run like hell out of the base and into the wilderness. If you make it the player is rewarded with the end of the alert and can make a new plan or phantom cigar back to normal if they have it. This is clearly to keep an alert from being overly punishing. Also, if your objective is in the base you have to go back in anyway so it makes sense to keep the majority of the defenses there. I don't think technical ability has anything to do with it. It's not hard to spawn a tank to follow your position, GTA has done that for years.
Oh and major bases have a shitload of enemies, go back and count how many are in OKBzero if you don't believe me. If its not at least 43 I'd be shocked.
Good analysis of the AI routines and how the cross-base communications and alert status, reinforcements, etc, add to the dynamism.
On at least several occasions I was waiting out an alert at one end of a base, setting up a sniper perch, when backup drove in from behind, spotting me just as things were starting to settle down, and forcing me to flee once more. Emergent moments like that kept me on my toes in the more densely guarded bases.