Please explain my confused me like I’m 5 (0r 4 or 6).
But you are missing the point,. There is no reason to ever start a calendar at year zero. The starting point can be zero, fine, but once the first day goes by, you are in the first day of year 1, not year zero and that is logical and has nothing to do with smart astronomers etc, “not understanding the number zero.”
At this point I’d say the only person who doesn’t understand zero is you.
It makes sense to start with the year zero when you want to do any calculations that involve dates that where before and after year one. If an empire was founded in 50 BC and dissolved in 50 CE to calculate its age when it was dissolved you have to acknowledge that there is no year zero so instead of just calculating 50 - (-50) = 100 you have to substract one which is counter intuitive. Because it went from year 1 BC straight to 1 CE.