They have instanced personal housing, they're called Personal Rooms. They want the main housing to be areas where people can hang out together, and instancing works against that goal. Besides, even if they gave people full sized housing via instancing, it'd still take up some of the servers capacity. They would need to remember that the house existed, what it's outside looked like, what the outdoor furniture placement was, etc etc etc. I'd rather they save that capacity and put it towards giving players more inventory space to work with.
The same problems exist with the personal rooms of course, but those are small enough that each doesn't likely take all that much capacity.
A room is a room, not a house. I have a personal room but I can't really decorate it like a house (not enough space and a low item cap). Also no gardering (soon fixed), not outdoor furnishing and no chocobo stables. So no, it's not the same.
The neightborhood system sounds nice and all, but really, can you tell me there's a lot of people socializing there? The neightborhood of my FC house is empty (and it's close to a market board) and all the wards I visited were empty too.
And even if they were not empty... let's face it, people won't socialize. They don't do it in Ishgard, in Idylshire... hell DF groups or FATE parties are silent as fuck 90% of the time. People socialize inside their FCs, but generally not with strangers.
Blocking people out of having houses for a false premise of people socializing is bad IMO.
And I also think you can't add a feature that is not going to be available no matter what for all of your players. After all, everyone pays the same monthly fee.
I know instancing will use server resources (adding more wards does too). That's why I said that it will take long. Other MMOs can handle it, why this one not?