The points at which the game transition between acts seem a bit arbitrary (mainly for Act I to Act II), and I don’t see a narrative or mechanical reason to lock us out of previous maps and quests. As far as I remember, previous Baldur’s Gate games didn’t have this kind of points of no return. Why do you think they did it? Do you like it?
I’m waiting for a mod to allow the back travel for my second play through, also hoping the level 20 mod gets updated to allow you to level your main class past 12