Because life is about looking back as much as it's about looking forward. Every year you live is just as much a celebration of having survived as it is an acknowledgement of being closer to death. We've had yearly celebrations for various reasons throughout all of recorded history, and likely well before it - it's ingrained in human behavior to seek patterns, and the seasonal cycle is one of the most recognizable patterns there is. So long as people enjoy being alive, we'll want to party when we recognize that we've made it one more cycle.