I don't think it takes knowledge of gravity/energy/entropy to generalize that things more naturally "fall down" rather things naturally "rise up". But, it's probably a stretch to say that influenced writing direction.

Others have made a possibly more relevant point - in one direction, your arm/hand will block what you have already written.

More languages read right to left than left to right despite most people being right handed, so the blocking what you’ve written thing doesn’t seem to add up either.

I agree human mechanics is likely the reason people tend to write down rather than up though. But I’d say it’s more about our muscles, we’re stronger pulling our arms in than pushing them out. But I’m no expert so would never claim confidence in my assumption there.