I feel like it was done this way to prevent RMT-related item/gil/whatever moving, but it's honestly been nothing but an absolute pain for regular players who happen to enjoy making alts (like myself).

Honestly I've completely avoided taking an alt of mine out of my private FC, despite wanting to try getting her in an RP FC suited to her background and goals, all because moving items or money between her and my main would be an absolute nightmare. I hate having to always ask a friend (or wait for a friend to be on) just to move one or two items at a time. ; ;

Even just the shared inventory slots idea would be a good step forward in eliminating an issue that's been prevalent for years.