I think I once read that they don't want to let us friend alts because of RMT and bots.
And they also said a few years ago that they at least aren't actually against us having our alts as tenants. I don't remember where I heard that though.
It seems like they don't really have the technical means to make our alts tenants. Maybe the game can't really "link" our characters, like they are completely separate in-game?

But I also hope we can share our house with our alts soon!