Except the combat system isn't as faithful to D&D as a turn-based setup is (and there are far better games in this respect)
This is a pet peeve of mine. Now I can easily see how the error arises for people who didn't play pen and paper AD&D (2nd edition) but ...
2nd Edition D&D was phase-based, not turn-based. You chose an action and
then rolled for initiative. The best way of representing this is exactly how BG (and BG2, IWD and PS:T) handles it. The game pauses at the start of the round, you choose actions for everyone, the game rolls for initiative to see how long everyone's actions take and they get executed in order.