It'd be unfair to all the players who DON'T have a personal house as they can only get more item storage by paying a monthly fee.

Sure FC's have this, but it's a shared chest, not for one person. Admittedly a person could level up an FC by themselves, but they're in the minority compared to people who have personal homes and that doesn't even need a home to access the chest so technically that's available to everyone right now.