No, new bandits don't just rock up five minutes latter to the ruins you cleared out. Why would they do that? "Hey, let's go live where everyone else was just killed!"
No, what happens is that the bandits have their own respawn system! Albiet not as neat and efficient as a Hyperion respawn unit!
This is why bandits wear masks - so as to (attempt to) protect their brains. Because they have cybernetic oxygen cells and a flash hybernation system implanted in them to keep their brain alive in case of their body shutting down.
Regrowth Units, Midgets and You!
This is why you get so many midgets! Because they are regrowing their body, obviously you get alot of bandits in the early, formative stages of growing a new body!
Psycho Melee Guys
And this is also why you get psycho's who just use melee weapons - all the chemicals and hormones from growing a new body are swirling around in them, driving them fairly insane! Like a teenager...just multiplied by ten! They just can't get it together enough to obtain and look after a gun. Sadly this can be a vicious cycle, where since an individual can't complete their growing cycle, they are time and again 'killed', repeating the psychotic hormone cycle. Eventually these long bouts of madness engrave their way into the bandits synapses, creating
Psycho Grenade Guy
Utterly mad and full of growth drugs, these guys can be conned by other bandits into simply weilding a grenade and running at you. Bandits both fear falling into this tragic situation, yet also glibbly exploit those who do. It's said it's possible to return from being a grenade guy, but most examples seem to be from word of mouth.
Crazy Bandit Attitude
Overall the bandits are affected by the hormones, even once they have completed the growth of an adult body. This, in conjunction with a sense that they can't die, makes their methods somewhat eratic and insane. Some may even suffer from dementia, as even though one might assume one is fighting bandits who are in their early twenties, some of them are actually seventy or eighty years old!
Growth Systems aren't just for People!
This is why you find so many guns in bandit toilets - the components are actually being grown by special bacteria! They gather and fuse molecules of metals in various ground up scrap fed to them. The toilet actually makes an ideal growing spot, being moist and...fed a continual supply of nutrients! Oh, sorry, if you were grateful to hear that the toilets aren't actually used in their traditional sense anymore, sorry! Indeed, quite the opposite!
As to why Hyperion, as run by Handsome Jack, the Handsome Jack who wants to kill you - as to why THAT Hyperion will keep respawning you????
I got nuffin'.
Imagination only runs so far...