So I disagree heavily with the claim that DND5e is not rich mechanically. But going past that.
Larian didn't just aquire the rights to work in the Forgotten Realms, if they wanted to they could have negotiated to make a new IP in the setting with their own rules. Instead they acquired the rights to BG AND the rights to make a game using the DND5e system (ostensibly with its addons since there are races from Volo's). In other words, they took on the challenge to making a game based on the 5e systems, that is what they chose to do.
Going back to my first statement though, 5e has a lot that can be explored by those who look into the mechanics. Character building is something I like to do, and I like to pursue random concepts and try to figure out how to make them, which almost always there is something in the mechanics or rules allowing me to make said character, especially with Tasha's and its changes to race and its addition of Artificer for more interesting combinations. And it isn't just in characters, Spells in particular are a highpoint of 5e in the variety and how they can be applied. True, in tabletop 5e rewards RPing and Creativity (effects like Minor Conjuration are practically built for it), but a straight mechanics game can function just as well as a RP heavy game. I will concede though that a large part of that is 5e mostly cares about Rules As Written and thus allows a player to be loose with most everything else. Going to minor conjuration, its applications are inumerable and thus is hard to translate to a videogame, but Larian has show great competence in translating such spells with Speak With Dead and its ability to speak to many of the corpses in the game, and so I believe that with otherRP focused spells that they will be able to translate many of them to a videogame while keeping their mechanics intact.
In fact, a game that I will not mention cause that will illicit a groan from some has been discussed many many times in this forum proves that 5e mechanically can translate to a videogame just fine and remain engaging and mechanically rewarding.
