A lot of the servers don't even have empty housing plots availableAnd unless they can actually keep their FC rank they'd have to recreate it and level it back up to 6 just to be able to buy a house again. Not to mention any members of the FC that have their own houses and have invested in the furniture/items that can't be removed once placed. Also the entire FC might not even want to move, they might be part of other linkshells/communities/raid groups that they wouldn't want to leave.
Combined with the gil limitation for moving server, and having to fire/rehire and re-level up all your retainers that you currently own, unfortunately I don't think many people would actually take advantage of a free transfer. Unless you're a fairly new character you just lose so many things by changing worlds.
Heck a lot of people won't even be able to keep the same name if they moved as well as their retainers when rehired would possibly also need to be renamed.