Image: A child carries a shofar. (With parents’ permission, all rights reserved.)
The short answer: tradition!
The longer, more complete answer: A long time ago, before we had scientific method to explore the “how” of the world, a Jewish scholar used the text of the Bible to count back to the date of creation. Then the Jewish community chose to number the years according to “the number of years since creation.”
By that accounting, the birthday of the world is Rosh Hashanah. If we baked a cake for the world, it would have 5780 candles. (And we would have a fire on our hands, I imagine!)
Today we have science to explain phenomena in the world, and Jews do not rely on the Biblical text for scientific knowledge. Instead, we go to the texts for questions of meaning: questions science cannot ask or answer. We explore the text to ask questions like, “What does it mean to live a good life?” or “What should we do about suffering?” Those are questions science cannot address.
So why continue counting the years from a date we are absolutely sure wasn’t the date of creation? The answer is simple: it’s our tradition!