Building floor numbers in at least a few countries I’m aware of start from zero or “G” ( or the local language equivalent for “ground“) with 1 being the first story above the ground.
I think you’re just biased to think that starting must “naturally” begin with 1.
Zero is just a good a place to start and some people do start counting from zero.
The floor number case arises so because traditionally it is the count of "built" floors. So, ground is technically not a floor in that sense. Also, if the floor indicates a separation (cut) between the living spaces, ground floor can be numbered as zero, just like the start point of a measuring tape is numbered as zero.
I think you’re just biased to think that starting must “naturally” begin with 1.
Zero is just a good a place to start and some people do start counting from zero.