SumofOne: It's a 5 year old game by now going for 6 why are they still so stubborn on keeping their online connection demands is beyond me. The sheep that buy anything you feed them and had interest in this game already bought it in consoles and Steam anyway.
The only ones looking to buy it now are the people that want a more honest products with true ownership instead of rental until servers are kept up. There is no reason to be so stubborn and not get money from those players too.
The game run it's circle by now, they got their money from those tolerating DRM. Just let a new free DRM circle start that will also guarantee the preservation of the work your developers did even after your servers are changed or down or your company closes.
do you really need an explanation? if your "confusion" if truly genuine, it is because you are using logic and "norms" to try and understand "policy". they (IO) are not doing what you want not because they technologically can not, rather because they do not want to - for reasons. reasons like if they do this for hitman 1, that will "kick the lie reasons' from under the remaining two and any future ones. they always stated things like: "we must use online features for many things in the game, to protect from piracy and to make sure scores and shit are legitimate" ..if they "now" (doesn't matter how many years later) make a genuine and OFFICIAL truly drm free version, just by doing so, it makes them "liars" or "great exaggerators" to many, including current customers of #2 and especially #3. as i am sure you know, many gamers around the world actually do like and prefer drm free games. however, most, decades ago now, "accepted" (mainly due to low will power) drmed game and services like steam. so "now" they are actually upset and outraged if/when a drm free game comes to gog (especially a AA/AAA type) because they then, among many feelings, feel they are missing out on the "best/better" version. so at that time, they actually "hate" others having "clean" copies and turn on the company doing that.
okay, so the above is one reason. another is that by doing so, they do and/or may give away certain drm/copy protection secrets that might/will make newer/future hitmans easier/quicker to crack. that is actually somewhat factual. of course if hitman 1 & 2 are drm free, cracking 3 has to be easier for serious groups. they can see what and how between versions. this is not new, and should not be a truly "good enough" reason, overall. this is one of those "shared" reasons.
another half reason (half, because it only happens IF they use "cloud" shit for SP stuff) is that once they go that way, they do get a lot of telemetry data that they would normally pay for years ago. you get so much customer data that you can even make money selling some of it. it can even become another "revenue stream". they often (always!) use reasons like; "hey this is really great for you the customer. if we see too many people having issues in a level we can make it easier." point being, that data collected from having/forcing ALL players require online accounts/usage, even if in some cases not initially for drm reasons, it then becomes too valuable to ever have offline versions again.
finally, and this has been officially (over time) stated by both IO and other companies making games that deliberately use online connectivity for single player parts and bonuses. they do it because they KNOW piracy exists and even their protected games are cracked. so this way, they want to show paying customers that :"hey, yes we know pirates are playing the game you paid good money for, but rest assured they are not getting the "full experience". they are missing out on _______" so if AT ANY TIME the official company releases a proper drm free version, their ENTIRE policy is revealed to be fraud or fraud like. it's a catch 22. if IO (et al) start embracing gog, they are immediately exposed as liars, frauds, shits, scumbags etc.
i know you know this, deep inside, but now you will not be able to ask such silly questions. and if you continue to act "innocent" that's on you. i'm actually more "peeved" with users who ask why, when in 95% of cases they know why, or they ought to.
tldr; the real reason some companies, those that love fucking single player content into cloud space, can not fully embrace drm-free, is because if they do, their entire company policy is exposed for the fraud and outright lies it is.