Here is the proof:
1)After the DF release - people been complaining about how empty the world feels due to sharding(source: look here and other forums).
2)During WoD - people been complaining about the same, as people would stay in thier garrison(personally, I remember how empty Orgrimmar was). Source: look here and other forums.
These are litteral and observable facts. And housing WONT help these issues - in fact it interupts WoWs social culture.
And no - you can't suddenly change the culture after 18 years. People won't start doing RPG stuff suddenly, just becuase you provide them with housing. It didn't work in WoD(while it's not the same, we all tested the waters).
Nobody mentions the maintenance cost vs. maintaining the rest of the game. It is technically expensive task to keep housing up and patched, while they have to decide how to spend the resources(Wildstar is another example of good housing vs. compromised gameplay).
But with your logic - let's implement housing in Fifa or CS. Because "improvements". Or let's have a soccer game in WoW - because of same reasons. Or let's add pineapple on pizza.
There are lots of people who can't accociate to housing, especially those who picked up WoW as their first MMO.
Blizzard would had it implemented long time ago, if it made any sense.