KneeTheCap: It's a shame that WoW, for instance, does not have any kind of player housing. Well, they have the damn farm, but that doesn't really count.
Really odd. There are roleplaying servers, yet they do nothing to encourage roleplay...
amok: They never had the technology for it from the start (like LotrO, where the homesteads is basically different instances), and it would now become to much of a hassle to implement. There is also a to big user base (10 million houses?), especially since many dedicated wow players have an arsenal of alts (I had 7 mains at a time....). It means there is a lot of space needed to run the housing.
It is a shame, yes, but technologically I can see how it would be difficult to implement.
EverQuest Next, on the other hand, is going to be built on a voxel engine, and it looks like players can build houses and towns there, thought it will still be very restricted.
edit- sprelling
Star Wars Galaxies, as I mentioned, managed to even have whole player-made towns built not only of player housing but also community buildings like a town hall, and not in an instanced area either, anyone getting near while on quests or whatnot could see the place (though only people with permission from the house owner could enter the houses, which were instanced). I was always jealous of that back when I played World of Warcraft.
Blizzard certainly could have added in-world housing (or housing at all) if they thought it a worthwhile effort.