Alt-Mailing

Apologies if this has been asked/answered previously.

Why are we not allowed to Mail to our Alt characters? Friends would quickly become annoyed being asked to help trade.
FCs discourage constant use of Vault, and it's valuable space.

I have 8 characters that I love and play, Aside from having to create my own FC, which would Alienate me from FC chat/friends, I really find it hard to trade mats/gear that my Alts craft with / wear.
I would greatly appreciate if allowing Alt-mail is considered. Thank you <3 :)

The direct answer is because you cannot friend an alt character on the same service account.
So there's technical limitations at play.

Though why nothing was done to resolve that? To curb RMT money moving is the most likely answer.