A guild is not likely to share an appartment building with other guilds, there is rivalry among many guilds and organisations for one. If we're talking about player housing, where the players only buy a single small appartment, yes then I can see the reasoning but not for guild houses. There it is all about stature and having something presentable .. this just wouldn't work well if we're all shacked up together.
Imagine the most ludicrous example you can think of ...
A Bounty Hunter just got a contract to hunt down some Outlaw. Finally his big chance to prove his worth so he straps on his equipment, checks his armor, weapons, food and drinks before starting his mission. With full determination he then steps outside of his guildhouse, in search of the ellusive criminal.
"Hi neighbour, how's it hanging?" says the Outlaw in question as he wanders into the very same building, off to his own guild's gathering spot.
The Bounty Hunter stands there for a few seconds, baffled ... until he finally remembers that the Outlaws are living across the hall from him. He rushes to his senses and inside the door, almost taking the door itself with him because he forgot to open it. Once inside he draws his weapons in an astonishing swiftness and hollers:
"Alright you criminal scum, surrender yourself or die today!"
Everything is quiet for a minute, then when the Bounty Hunter's adrenaline rush fades he sees a room full of people staring back at him. All his fellow Bounty Hunters are looking at their comrade with their mouths wide open, thinking one of them has gone insane.