The sticking point, continuity-wise, is that the producers of Enterprise
opted to include a ready room in their sets. With good reason, because it is
a sensible narrative device for the captain to have an office right next to the nerve center.
My theory is that (for whatever reason) the original Constitution classes simply weren't built with this in mind. Maybe there was a change of attitudes on the matter of captain's being nearer the bridge compared to Archer's day, which was later reversed?
The updated bridge set seen in The Animated Series has of course got an extra door to the left of it, which Roddenberry always said led to another Turbolift. But I've always liked to imagine that it led to an airlock area including an emergency Turbolift and
maybe a ready room for the captain (even though we never actually see him use it). Of course, this doesn't cover why the bridge sets in the movies don't have such a door...