100% agree it's actually crazy how detailed that game is. Not just in its sandbox interactions but the writing is far and away above anything else.
I don't know what Rockstar's process is for writing but I get the impression story writers have significantly more power and impact inside the company than in other companies where they're sidelined by level design and programming. Something significant must be different about their internal structure to cause the difference too because it's been consistently improving over time and that must include staff turnover.