There are three arguments that I can think of, but I wouldn't say that they are against being closer, but rather as arguments for expanding 5e.
First - 5e simplified previous edition rules (at least compared to 4e) to make the game easy to get into, play and run. One of these was Ongoing Damage, which in my opinion was thematic, but a pain to keep track of, and I am glad 5e got rid of it. That said, in a video game this is not that much of an issue.
Second - kind of the same reasoning as the first one, but this time it is to do with 5e being a tabletop. I don't know about you, but most maps I play/make tend to have very limited verticality, because it is rather confusing for all parties involved. Sure, a few flying creatures are ok, but it often confusing for players. So adding some sort of mechanical advantage to verticality beyond what distance gives you would be hard to keep track. Again, in BG3 you are not limited in this so you can take full advantage of 3d environments.
Third - the game being mostly about combat. Sure, exploration/social exist to a degree, but BG3 can never rival 5e. This means that certain class features/spells need to be adjusted. For example PHB Ranger features are mostly pointless in BG3 (they are bad in 5e as well, but you could make them work). Utility spells like mending, shape water, etc will be dumbed down even if they make it into the game. And the problem with these is that most people will have a different idea of how they should be dumbed down.
EDIT: incorrect 4e reference
Last edited by Eugerome; 03/11/20 10:04 AM.