Making targets fight back has traditionally created a lot of problems.
Some of the targets in Codename 47 felt more like traditional video game bosses than people who you were going to assassinate and there was a comical difference in difficulty and engagement between sniping Pablo or Lee Hong vs engaging them one to one.
Silent Assassin had the problem that while some of the targets did fight back, they were woefully unimpressive until you ran into Sergei who suddenly could soak multiple shots and could fight on even after being chloroformed or strangled (which was admittedly hilarious)
In Blood Money you could lure out some of the targets from behind their security by aggravating them (either by doing a tiny bit of damage or just letting them see you trespass) and let you assassinate them. It’s a common strategy in speed runs and occasionally hilarious, but did ultimately undermine the rest of the work in level design, etc.
Lastly, if we just limit ourselves to the latest iteration - there would be a lot of discrepancies and potential confusion over which characters fight back and how effectively. Even on say, Colorado.
Sean Rose is an amoral bomb maker - so would he know how to handle himself in a gunfight? Maybe, maybe not… most of his work means never getting up close with people so he might not even know how to switch off the safety.
Ezra Berg is a former Mossad agent, which means he was probably in the IDF and almost certainly has some military experience. He’d certainly be able to shoot back but given he’s working as an interrogator on a heavily protected campsite - would he even bother having a carry piece?
Penelope Graves is a former Interpol profiler… so maybe she has some combat training from a police academy, but like Rose it’s quite possible she never bothered and since she’s the suspicious outsider with a pair of bodyguards - there’s a decent chance she’s not allowed to have a carry piece.
Maya is a former pirate and insurgent, now missing one hand, and has by far the most combat experience and aggression in combat of all the targets… but is limited by having only one fully functional hand. May also not have a carry piece.
If the game assumes that none of them have carry pieces and all of them are shocked by the attack, they can use all the same behaviors and aspects. Otherwise having all targets who fight back doing it the same would be just as jarring for the player (“why is a general who’s never actually fought in a battle as badass as a grizzled ex-secret service guy?”)
If the game were to try to incorporate their characters into self-defense… that becomes a nightmare for working out how to work it, how to balance it and how to deal with player’s expectations.
After all if my only experience with letting targets see me is they run away to get help, then cower when cornered - I am probably going to build a plan around this and may get quite annoyed if after fifteen minutes of planning my target responds to an explosion by pulling out a gun and charging toward me instead of running into the room where I’ve set up a proximity bomb.
It’s one of those things that works great in imagination but creates a whole bunch of potential problems when you factor in the complexity of the game and how many loops and plans it could obliterate if a gun goes off at the wrong time.