In Game Mailing System

[Cigaran]

the system is also in place so you can't trade between alt characters making characters and accounts forced to have to friend each other to trade money. (basically a way to see where the money is going)

They can see where the money is going without the asinine restriction of not being able to move gil/items between characters on the same account. Hell if anything it makes it EASIER to track and deal with since it's not involving additional accounts.

[Orophin]

I think it's like this so you have to add RMT for them to mail gil to you.

That's why they offer 100% Handworl instead.