Cool. So why don't you celebrate the birthday of Siddhartha Gautama? Or Confucius? Genghis Khan? Julius Caesar?
Because Jesus was not the most influential man in the history of the world. He was influential, but the world is bigger and history deeper than Christians want to believe.
Hell, the only reason anyone even still cares about Jesus is because the Roman emperor Constantine decided Christians were more valuable to him fighting in his army than being fed to lions. If not for him, Christianity would have just been another weird Levantine cult and would have probably died out. So why not celebrate Constantine's birthday instead?
Or the Apostle Paul's birthday? It was his idea to spread the faith to gentiles after all, and one could argue modern Christianity is more influenced by Paul than Jesus.
This is not an opportunity for you to proselytize me.