While this usually a PC game basic, I think the decision to not add it is due to the ease of running third party programs while FFXIV is running.

Anyone who wants to run a third party program knows how to work around the lack of ALT+TAB, so they are mostly inconveniencing their more casual players.

Of course, everyone has their old computer sitting around anyway. ALT-TAB would be nice, but then that old PC would truely be worthless.