The question is in the title of this tread. I asking because of curiosity. I already own a motorized 64mm flat burr grinder (SuperJolly) and manual/hand 68mm conical burr grinder (Pharos).

I apologize in advance if they or many exist. I did try searching topic first before this post.

Good explanation of inherent design problems and R&D and marketing:
Making a hand grinder with flat burrs

Why one likely not exist or if they do why there would be rare:
Is it not possible for a manual grinder to use flat burrs?

No, the dynamics of a flat burr grinder argue strongly against armstrong power. After all the complaining about the difficulty in cranking the early Lido-2 and Pharos you would not be able to handle the racket a flat burr grinder of that size would cause. Would be an instant fail in the market place.

OldNuc, the country living grain mill and our own Jim Schulman's review of it might argue against your assessment of 'the dynamics of a flat burr grinder'
Titan Manual Grinder

Please note the length of the lever arm and the design of those grain mill burrs. You can make decent drip grind coffee in a grain mill, maybe. There are no flat burr manual coffee grinders on the market. The horizontal shaft, vertical flat burr mills usually are clamped to sturdy tables.