Why do unfinished wooden utensils have a different law than unfinished metal utensils?

Question

In Keilim 12:8, we see that unfinished wooden utensils with a receptacle are susceptible to tumah, except those woods whose properties make the utensil unusable.

‮... כל גולמי כלי עץ טמאין, חוץ משל אשכרוע. רבי יהודה אומר, אף גרופייות של זית, טהורה, עד שתישלק.

... All unfinished wooden vessels also are susceptible to impurity, excepting those made of boxwood. Rabbi Judah says: one made of an olive-tree branch is also clean unless it was first heated.

However, in Keilim 12:6, we have a disagreement as to whether unfinished metal utensils are susceptible to tumah.

‮ארבעה דברים רבן גמליאל מטמא, וחכמים מטהרין... וגולמי כלי מתכות...

There are four things which Rabban Gamliel says are susceptible to impurity, and the sages say are not susceptible to impurity: ... and metal vessels which are still unfinished ...

Although Rabban Gamliel's opinion in 12:6 works well with 12:8, the sages' opinion does not. What are ways that commentators have tried to resolve this?

Answer 1

This question is asked and answered in the Gemara in Hulin 25a-b. A Sefaria explains:

מאי שנא הני ומאי שנא הני?‏
רבי יוחנן אמר "הואיל ולכבוד עשויין"‏
רב נחמן אמר "הואיל ודמיהן יקרים"‏

The Gemara asks: What is different about these unfinished wooden vessels , with regard to which the halakha is that provided they are fit for use they are susceptible to impurity, and what is different about those unfinished metal vessels, with regard to which the halakha is that even if they are fit for use they are not susceptible to impurity until their crafting is complete?

Rabbi Yoḥanan answers: The difference is that since metal vessels are crafted for uses of honor, they are not considered vessels until their completion.

Rav Naḥman answers: The difference is that since the worth of metal vessels is expensive, they cannot be sold at that price unless they are complete.