Except, it's pretty obvious that the vast majority of people were not playing the game in 2020. A minority played the Early Access, yes, but even then, that was only less than a third of the game. You couldn't glean all of the plot points of BG3 from that, and without those, the ways in which the book fully ties in with the video game are not clear. For instance, at some point you learn that the Dead Three are behind most of what's happening in the game. The Dead Three also played a role in DiA, but their role was secondary to Duke Vanthampur and his infernal plots. I'm at the start of Act 3, and I still don't know if the beginning portion of DiA and the big plot in BG3 are related or not. Without clear plot threads creating a synergistic story between the two games, it's not a good tie-in. And we didn't have access to BG3's full story until two weeks ago.
So for all intents and purposes, BG3 came out this year. A small minority played the Early Access, but the buzz they generated is negligible compared to the new crowds that are playing the game now. The smart synergistic strategy would be releasing DiA now, not three years ago when a (at the time) dubious Early Access programme was launched.