Because if they reply every emails with archives, they lost important time. That time they use it to make virus signatures quickly(ASAP). What you think is more important make virus signature ASAP or reply emails?
I’m well sure that they are taking care of your possibly infected file. Sometimes they won’t have time to reply to you but I’m positive that they’ll put it on the next update or not.
When I was McAffe and AVG user, they always made both: update data files and sent email with information about virus or not in the received file(s).
It could be a “robot” email: Thanks for your e-mail… we are checking… etc etc…
and if it is a positive, another “robot” email: attached file in your email is positive, please check new virus-signatures, or something like this.
Two tasks in keyboard pressed and 2 emails sent. “IT” makes miracles.
It’s not so much time as you think. And a good image for the company.