Why must you say anything at all? This is a centuries-old meme that serves no purpose. It annoys me slightly anytime says it to me, and I don't say anything to anyone else when they sneeze.
It's a behavior that we as a society have decided is polite. It hurts no one, aside from maybe a few neckbeards who have entirely too much invested in Being An Atheist (or the
equivalent for your hair-splitting worldview of choice).
That you choose to get "annoyed" about such things is your problem, nobody else's.
I avoid a response after someone says it. Most of the time, I do not think they notice, but sometimes I get a "Why do you never say 'Thank you' when I say 'Bless You!'" from a person. My response is that I would be thanking someone for doing something for me, but the person has not done anything. If saying 'Thank you' was a polite response to me saying something according to the rules of society, people should be thanking me when I say hello. I do not say "God Bless You" or "Thank You" in response to this. I must agree with you that the meme is purposeless. I have no trouble with people saying "Bless You" in any way, but it should not be related to something mundane such as a sneeze.
