I say gesundheit. Fortunately, I live in an area with a lot of Germanic last names. It is very common here, even amongst some religious people. I once lived in a more diverse area briefly, and some people looked at me like I was from Mars - what is this word? (They also had never heard of snickerdoodles.)
On the receiving end ... that's a different story. I have some allergies that are triggered by many smells. I sneeze often. I can't begin to tell you how damn blessed I am! A few times, I have had conversations with coworkers who, sometimes on a daily basis, blessed me over & over. Sometimes I have told the person that I sneeze often and have been "blessed" so many times that I will not take offense if she doesn't stop working to say it. Other times, I have pointed out that we all know my soul is not escaping when I sneeze, so I don't need to be blessed - and since I sneeze often, it is annoying for both of us when he feels compelled to say something at all. (Success rate - pretty fair.)
I forgot the rest of your question... why?
I say it because it is socially expected where I live to say something.
When I lived in that diverse area, not nearly as many people said anything at all. So I stopped saying anything. (Yay!) I would only say it if others were already saying, "Bless you." Then I would chime in with my, "Gesundheit."