A piece of media should stand on its own, especially if it's not a sequel. And Baldur's Gate 3, regardless of the "3" in its name, is not a sequel. Not to BG1&2, and not to Descent into Avernus. Those stories began and were completed in their respective pieces of media. BG3 begins when Descent into Avernus ended (and years after BG1&2), sure, but it's not a continuation of those stories - plot, characters, themes, etc.
All of the rules and lore required to understand, play, & enjoy a video game (to the extents the developers intend you to) should be present within the game. Or the developers need to come out and explicitly state "you must have played A and/or read B in order to get the best experience out of our game." (It's still a shittier game with that warning, but at least customers are forewarned)
