One factor no one has mentioned (as to why you don't see any PRODUCTION 12 string arch tops)....
Generally, players of upper end arch top guitars do not (as a consumer group) desire 12 strings instruments.
If there was a "want and need" for these instruments, some company would have developed an instrument to fill that demand.
This is a case of "WHY" having nothing to do with the technical merits of the instrument style, but instead it's a case of no market niche to fill. Of course, there will be one-off exceptions but that's EXACTLY what small shop/custom builders are for :)