Because if you reply in any form that's not a flat out rejection, the fuckers keep emailing you back. So you end up in a penpal situation that you dont want.
And if you tell them your not interested, you get a nasty flaming reply and they bitch on the threads that you rejected them for whatever reason. Especially if you are Caslon.
It's better to just let them wait til they realize that you arent responding. Hopefully, by then they will have gotten over you and started emailing somebody else.
I think it is rather cheeky to think that an email from you obligates anyone to respond. Rather you should feel complimented if you get a response, but not all knicker-twisted if you don't. It's just like if someone knocks on your door or calls you on the phone. You are not obligated to answer either.
And I'll bet that if we ran this electronic epistles past the proverbial Emily Post, she would shit bricks.
If you're gonna get so totally pissified if you dont get a response, dont write in the first place.