Now I'm no programmer, but I don't really see how hard it is to just take the players handbook and start implementing the ruleset as is?
It's hard the CHANGE an existing game engine design, because the AI has to be programmed to use whatever rule set you use. Right now the AI is programmed to use height advantage, deal with a plethora of surface effects, and every other "homebrew" change from 5e in the game. Any change to how it's working now, means reprogramming those AI routines. Basically, rebuilding the computer DM.
This is why it's hard to make a difficulty settings menu where you could choose strict 5e rules or the current Larian mode. It's much easier to program different difficulty settings by just altering HP, AC, and a few other variables like reducing the spread between dice rolls for Advantage-Disadvantage. That doesn't require reprogramming the AI for a different rule set.
BTW, I'm not making an argument that the current design is better than a more faithful 5e adaptation, just pointing out that it's not simple to switch between systems.