Basically, what we're asking here is, why wasn't Kirk's office right next to his bridge, like everybody else's? And the answer might be, it was, until it was taken from him.
Picard had various conference rooms available, too, and frequently used his cabin as an office of sorts. That didn't negate his need for a private room from which he could dash to the bridge in a matter of seconds - indeed, his big negotiation table was on a room that was likewise right next to the bridge. This is quite practical, as a starship performing a mission would be doing the good old "ninety-nine minutes of boredom and one of sheer terror" routine, and you could never tell where that one terrifying minute would fall.
Now, space on ships like Kirk's was no doubt at a premium. All the more reason, then, to put Kirk's office where it can double as his ready room.
Perhaps that's where it was? In "The Cage", Pike appeared to have a cabin of an unusual shape right next to the big circular briefing room, or a short corridor length from it; the walls and shapes rather suggest that both were directly beneath the bridge, in the superstructure of the ship. Kirk supposedly flew the very same ship, and quite possibly was supposed to use these facilities like Picard used the Observation Lounge (*) and the Ready Room. But something happened; perhaps the ship was refitted and these spaces had to be sacrificed, or perhaps there was damage to these decks that Kirk never had the time to get repaired. When we next see him, his cabin is down on Deck 12, then perhaps on Deck 3, and once on Deck 5.
So the answer might be, any Constitution
skipper other than Kirk the wandering gypsy
had a Ready Room and a Conference Room right next to the Main Bridge (one deck down).
(*) That facility was actually supposed to be called the Conference Room or Conference Lounge; Observation Lounges were random sets decorated with 1980s lounge stuff, as seen in "Justice". But at some point, the room next to the bridge was incorrectly addressed in dialogue, and the terminology stuck. Other designations suggested for the facility in preproduction included Ready Room (when Picard's private nook supposedly went by the name Away Cabin)...