Because it takes 365 days for the Earth to rotate 360 degrees around the Sun. It's a physical fact independent of man made calendars.
Actually it takes more like 365.25 days. That's why we have leap years.
But 365.25 is just a rounded number, so we have to compensate a little further. Every 25th leap year is cancelled, unless it is a 100th leap year in which case it is not cancelled.
Do not the unbelievers see the fingerprints of divine creation and order in the creation???