I have wondered about that limitation for at least 15 years. I
have come up with possible explanations but without a shred of
evidence from the RFCs.
FWIW,
The construct of "group" is pretty much an assertion of an aggregate
identity. That is, an identity beyond that of the listed addressees,
deriving from their, ummmm... grouping.
The purpose of From: and Sender: is to assign bits of essential
responsibility.
Unlike the current trend towards declaring personhood for corporations,
the design motivation behind using the group construct was to restrict
assignment of responsibilities to individuals, rather than aggregations
of them.
All very abstract, I admit. From the vantage point of some decades,
even quaint.
d/
--
Dave Crocker
Brandenburg InternetWorking
bbiw.net