In their defence, adapting code to a completely different engine (and API) can sometimes be more complex than developing for an engine developed in-house, if the structure of the interface is not compatible at all.
And instead of creating away, they have to see what's there, understand 15-year-old code and scripts, see the logic of the whole game, and mostly redo everything in a similar way.
The System Shock remake is based on the Unreal Engine - and they're not looking at old code or scripts AFAIK - as they have no reason to do so. They're using the modern engine to recreate what the game looks and plays like - which has a bunch of great editor tools, and they don't have to recreate much in terms of the rendering code, as Unreal Engine can already do every single physics or rendering engine-related thing that the original game could do - and then some. In fact, I doubt they've done a single non-gameplay line of code - except perhaps for some of the dismemberment stuff, which is arguably gameplay code.
Heck, a bunch of the gameplay related code should be more or less ready to go with modest implementation work - as System Shock is basically 90% shooter with light RPG elements. Developing a shooter in the Unreal Engine shouldn't be insurmountable.
That said, obviously it will take work - and they need to implement a bunch of RPG-like mechanics and the AI stuff. Same goes for the Cyberspace visuals and movement code - and the various toys you get in SS. Still, really not as much work as one would think - as the engine is fully capable of it all without having to make it possible first.
The art is more demanding today than in '94 too.
Well, yes and no. Remember that the tools were very, very different back in 94 - and creating textures and meshes for a 100% custom engine without ready-made tools was very time-consuming as compared to today.
I mean, I know from personal experience how much you can accomplish with these modern developer-friendly engines as just a single person. You can literally download the vast majority of asset work for a modest cost - and import them directly into the engine. This includes stuff like animation, textures, meshes and sound.
Obviously, much of that will be hand-crafted here and not imported (at least I would hope so) - but the engine is definitely ready to handle asset imports with surprisingly little work.
Now, if the System Shock remake was an AAA game with that level of production values - they'd need a much bigger team to create highly detailed assets.
But the game is actually quite primitive when it comes to textures and assets.
I don't know if you've played it - but it's most definitely not an AAA kind of game. It's very clearly an indie title with a limited budget.
No, I'm afraid this Remake is amateur-hour. Either it's basically a one-man team, or they don't know what the hell they're doing.
Not too surprising, though - given that this is their first game of their own. Well, except for all the design work and concept in itself.