Gcode exists because hardware abstractions are even more leaky than software extractions. When pixels go onto your screen, you don't assume to know better than the guy who wrote the driver for the graphics card how they should get there. When plastic gets deposited on a 3D printer the way in which it is deposited actually affects the properties of the resulting object. Same for a CNC lathe or milling machine, although to a lesser degree.

There are of course also historical reasons, when it would be a central mainframe that would generate the gcode, and then it could be executed many times by cheaper computers attached to the machines. There was even a point where a lot of gcode was written, or at least edited, by hand. In these modern days of compute excess, gcode probably wouldn't have developed to the extent it did, and we'd be distributing STLs with some metadata around tolerances, materials and primary stress directions and the machines would figure it out themselves. The equivalent of gcode would just be used as a communication protocol between the interface and the motor controllers.