As far as I know, the ancient Mideast used wet clay tablets far more than chiseling. I don't think anyone was as big on chiseling tablets as the ancient Greeks, who wrote from left-to-right.
Also keep in mind not all Semitic languages were written from right-to-left, notably Ethiopic and Akkadian, the latter which also encompassed the major dialects of Babylonian and Assyrian, all of which of course share the same ancestor
as the "backwards" Hebrew and Arabic.
I think any speculation about this is rather frivolous. I'll say for three reasons I don't buy that chiseling from right-to-left being easier is the reason for the direction -- plenty of other people who wrote left-to-right chiseled, chiseling was rare compared to impressing wet clay, and before you begin, as a society, to chisel you need an already well-established script (with direction).
Why isn't anyone equally curious and readily speculative as to why IE languages are written left-to-right? And the vertical scripts of Mongolian and old Chinese and Japanese? Don't forget to distinguish between left-to-right vertical scripts and
right-to-left vertical scripts. As far as I'm concerned, it is because it is, and the only answers that will satisfy anyone will be oversimplifications.