> Although it is hard to pinpoint why Arabic is sinistroverse, it has been theorized that before the papermaking process emerged in China, the Arabs used chisels to engrave the ideas and concepts they wished to manifest on stones. Due to its longevity, stone was preferred over leaves, skin, bones and shells. Considering most of the “writers” [and individuals in general] were right-handed, they would use their right hand to hold [and carve with] the chisel and the left hand to hold a hammer. Such direction is more natural because it involves an outward motion and is, therefore, easier to perform from a motor point of view. A left-to-right motion would have been counterintuitive, especially with the risk of personal injury involved.[1]
However, as another comment mentioned there are techniques that solve the smearing and awkward hand-position by rotating the paper. I would not be surprised if those are taught more widely for Arabic as 90% instead of 10% of writers face that problem.
[1] https://www.pangeanic.com/knowledge_center/arabic-right-to-l...
So any hypothesis about the origin of the right-to-left direction must discuss only how the tools were used for the Egyptian hieratic (written with brushes) and hieroglyphic (carved with chisels) writing systems.
Maybe it was easier to carve hieroglyphs in that direction, but I am not aware of anyone testing this hypothesis in practice.
If you hold the hammer in the right and the chisel in the left, you swing the hammer from the right to the left with the chisel tilted to the right. Thus the natural writing direction (outward from the swing) would be right to left. Swinging your hammer from the right and moving to the left would mean writing in the opposite direction of the force used to write single letters.
So, if the hammer is in the right hand and chisel in the left means a right-to-left motion and makes a left-to-right motion counterintuitive.
Think of it as a nail: if you need to hammer in a nail in exactly the right place, you put the nail into exact position holding it with a left hand, then strike it with force using hammer in the right hand. And if you ever did this you know that the tricky part is hitting the nail the right way, so you don't bend it.