I think it's both server costs and wanting it to be an exclusive feature, hence why housing is set in wards and doesn't just send you to an instance with your own plot of land.

Quote Originally Posted by Remolia View Post
So these limitations were made on purpose, to make housing exclusive on purpose, not coz its limitations of hardware/software. People can simple open a dutyfinder dungeon for one player, anytime. And have own rooms.
This is false, only a limited number of instances can be opened at a time. While it is fine now, when population spikes up—such as when a new expansion launches—all the sudden queue times and errors exist for things that it normally shouldn't.

"Raubahn Extreme" is a great example as it was an early Stormblood solo duty that bottle-necked everyone due to the limited number of people that can be doing it at a time. It's why Stormblood and Endwalker splits the population into two zones early, does not have any solo duties and takes a long time before you get to the first dungeon.
Gold Saucer GATEs have no role or party requirements yet sometimes you'd be waiting a while to get into Leap of Faith or Air Force One.
Trying to enter your house would often times give you an error because the housing instance servers are full.