The machines I worked on had air bearings. No wear. Basically we're talking about something like an error of 10um over 2 meters. It's practically impossible, or cost prohibitive, to remove that error without calibration. The granite surface the air bearings ride on is locally flat but not perfectly accurate over its entire length.

I agree this setup is almost perfectly straight and flat. But it's still not accurate without the calibration.

Just to be clear, there's weren't milling machines...