By that logic the only non-arbitrary dimension is defined by gravity, so the primary axis (X) should be up and down.
Math is not rigid like you are saying: 3D coordinates can be oriented in any direction because they are fundamentally arbitrary. A lot of people a damn lot smarter than you have damn good reasons for using different coordinate systems, and they are mathematically correct.
By that logic the only non-arbitrary dimension is defined by gravity, so the primary axis (X) should be up and down.
Math is not rigid like you are saying: 3D coordinates can be oriented in any direction because they are fundamentally arbitrary. A lot of people a damn lot smarter than you have damn good reasons for using different coordinate systems, and they are mathematically correct.