Originally people said "bless you" after someone sneezed because it meant that you had the plague and would die.
I still say "oh my god" and "if god wills" as they're just forces of expression. Just like our tendency to say "literally" and other wrong things.
Around here we just shout "Health!" when someone sneezes. ...It sounds a lot less weird in my mother language, somehow.
there are a :red: ton of languages in which people respond wishing health when people sneeze https://en.wikipedia.org/wiki/Responses_to_sneezing