Too many gameplay restrictions

[Cidel]

and 4... But then it would be harder for them to sucker you outta more money for extra retainers, so why make better quality of life changes when they can make a profit.

This is probably the real reason right here.

Letting your main character send items to any alts would directly conflict with the cash grab for buying more inventory space with additional retainers.

This is a feature we likely won't get all, if not without (surprise) heavy restrictions and limitations on what can be accessed by alt characters.