So for example lets say someone started an FC and everyone got together to make it happen, chipped in gil and all. Then suddenly the leader boots out every single member but him/herself and converts the FC house into a single player owned house with all the things inside and no consequence whatsoever.
This is why they would not allow the option you are putting forward, even if they were to add it there are very few people with your situation and there would have to be a great many checks and balances to prevent the aforementioned abuses.
Yet the option to convert the house would let other people share the cost of the soon to be personal, thus is not a great plan
I presume land is at a premium still and they just don't want to start over, but for every honest person out there is another who would use this for evil.