Have any previous DnD video games stuck completely to the rules? I'm not aware of any. I'm not sure how many DnD TT player groups stick to the rules either.
Even though the game trades on DnD 5e and Baldur's Gate/FR as hooks, I don't really expect it to completely correlate with either. Regardless of how strongly you feel about the mathematical integrity of 5e rules, I think that is probably less important to the game's developers than the play experience matching what their core audience would expect.
The central demographic of video games players likes high quality A/V, likes things that explode, likes Marvel/DC like super-heroics, etc. The game is designed mainly for that audience, and probably needs to be for financial reasons.
I don't really see Larian changing the core game design prior to release. Neither should they, since those that have purchased EA could see exactly what they were buying from the early gameplay videos; it has not changed.
As with most of the more fundamental game changes that different players want, complete 5e accuracy is more likely as a post-release optional mode or from the mod community.
