Part of the problem with housing sweeps is that even though the original purchaser may be long gone, other active characters may still be resident there, so it gets kind of mucked up, as we don't just want to yank houses away that are being actively used.
As for willed houses going to characters on alt accounts of the same player, that generally doesn't happen. We're pretty aware of most alt accounts and while there may have been the odd exception to this, generally we don't pass houses from one character to another played by the same player.