Because the dating system is not from the beginning of creation. It is 2016 years after the year traditionally held to be the year of Jesus's birth. Calendars which do use a dating system that stems from the year of creation, like the Hebrew calendar, maintain the year out of a tradition and convenience. Calendars are about determining what year it is, not determining how many years it's been since the beginning of something. 2016 could be a random-ass number and it still works because we know how many years ago the moon landing happened.