Menachot 34 - Why Write the Mezuzah on a Scroll

Since the Torah said, "And you will write them (those words) on the doorposts of your house," you might think to write them directly on the stones of the doorway. However, we learn otherwise by comparing this writing to the writing of the divorce document, Get, which should be "on a scroll," and this teaches us that the mezuzah is written on a scroll, and is then affixed to the doorpost.

But perhaps we should rather compare it to the writing of the words of the Torah on the stones, as the Jews did when on their entry to Israel? - We prefer to compare writing that applies in all generations (mezuzah) to writing that applies in all generations (Get), and not to writing which only happened once.

A better derivation is from the words "and you will write" (uchtavtam) themselves, which can be translated as "complete writing" - that is, on the scroll, and not on the stones. Now that we have this direct derivation, what do we need the previous one for? - You might still think to engrave the words on a stone, then attach the stone to the doorpost. This teaches otherwise.

Art: Laurent de La Hyre - Moses and the Tablets of the Law