Because that only makes sense if you want them replying to everybody else, which is almost never the case, or you'd just make a real mailing list. I'm not saying "don't send e-mails to multiple people" I'm just saying "use CC appropriately."
I, for one, would be safe using e-mail this way: none of my relatives even know where the "Reply" button is (though they long ago located Forward, sadly.) They just create a new message addressed to the original sender when they have something to say.
