Left-to-right writing systems are optimized for right-hand use. Two examples:

* if you're left-handed, your hand smudges over the ink before it dries. There are various contortions that some left-handed people do (hover the hand or wrap it around from above) - right handed ones don't need any of that.

* stroke patterns, as usually learnt in school, result in pushing away if left handed, vs drawing to, if you're right handed. This results in less ideal strokes, and if you're working with a sharp pencil/pen on a sensitive paper, this can tear the paper. If you're working with a felt-tip pen, the line width/pressure suffers as well.

That said, if you really make an effort, you can have a pretty decent handwriting if you're left handed. And if you are forced to use right hand when learning handwriting, you can still have a pretty decent handwriting.

I'm not familiar with details of chinese handwriting (what's easier/better if you're left vs right handed), wouldn't be surprise the constraints are similar.

So I guess your remark about messy handwriting is related to the strict standards for the students (which includes expectation they must write with right hand).

Due to a broken right-hand, I had to write with my left for 3 months and noticed that our alphabet is made for right-handedness. That's why I agree with your take that writing with the left hand is basically unnatural. But since typing is more important than writing nowadays (or am I in a bubble?), I don't think students should be guided to write with their right hand.

Seems probable that's simply because it isn't tolerated as a choice.

Though the best evidence to refute "There are no left-handed in China" is that it didn't take long to find a left handed Chinese baseball player

https://en.wikipedia.org/wiki/Chen_Hao_(baseball)

In many parts of Asia they will 'correct' children who are using their left hands.

That's only meaningful if the messiness of the handwriting correlates significantly with the handedness.

(and none out of thousands seems statistically unlikely: China has lower numbers of reported left-handers, but it's 3% vs 10%)

My father was left handed like me and he got in trouble from teachers.

It's possible that Chinese will one day obtain individuality and freedom and they can write left handed. That would kill the one last advantage the West has.

In my own personal and subjective experience, the correlation between left handed people I know and "edginess"-level is almost 1.

I am inclined to believe this is a learned trait rather than an innate one (excluding the obvious reasons why one would be left-handed only).