Because the only endgame incentive other than raid gear is glamour, mounts and housing. The game constantly throws at you furniture, walls, outdoor furnishings, materials etc., and it does it through retainers, primal fights, raid fights, treasures, dungeons, pretty much everywhere, skipping all this is your choice, but there are some that me that would've liked digging in. Granted, I'd buy a house even if there weren't all the existing features locked behind this system, I've always liked moving furniture and decorating small towns/gardens/houses (I loved Dark Cloud 2 and the construction minigame for instance), but it has got to a point where I'm inclined to think that people are just buying a house for the morbid feeling of seeing how exclusive they are and how many people are willing to sell their souls for one, because when you get to see the effort that the average player puts into decorating their house, you know they don't even care much about it, they just need to feel they are in possession of something exclusive. I know I spent hours and hours and hours decorating the garden of my house before losing it due to inactivity.
I'm fairly convinced housing demand would be way lower if there were infinite plots and everyone could have access to one.
