to the best of my knowledge you have to mod the PCBs to get stereo out, which disables the usual Jamma sound output.
they didn't ship like that, even if the sound was programmed for Stereo in some cases.
99% of arcade games used mono, because that was the Jamma standard.
that's why they're mono in MAME at least.
in the past a lot of things were incorrectly set to stereo.