again it a US show. which is why they placed it in the US. This was mention in TOS and TMP.
Now if the Preidents office was shown in TOS it would have also been in the US.
It was only beacuse everything went PC in the 80's and the fact that the show was big internaly. (Not sure on Galatic, will find out when we get out there.
) That TNG had the President office in Paris.
Now if you look at today armed forces. their HQ are full of paper pushes. Yes the Joints Chifs has an office there. but they don't spend to much time there as they are either in Parlament/ Sanate. or traveling around their Country or places of opperations.
If we take that into account with ST, the head of Starfleet and the heads of the different area of starfleet would spend most of their time in Paris in the Federation Govermental building. if they where not there. they would be at the main facuilty of their area. Medical, science, engineering and so forth. They would spend maybe one day a week at Starfleet HQ, or given the size of the federeation and the time need to travel between planets, it could even be less time.