The reason is that Job was not dictated to Moses by Hashem for the purpose of being put into the Torah. The words of the Torah were specifically for the history, halachos, and hashkafa of Bnei Yisrael. Thus Moshe wrote it at the lower level of nevua set up for Kesuvim.
The Chumash is like the Neviim in that they were given as a message by Hashem to the Navi with instructions to repeat them to Bnei Yisrael. They were then written down (also as instructed by Hashem. That is why we see Vayedaber Hashem el Moshe Laimor constantly. Just as Moshe was given the prophecy, he was explicitly commanded to tell everyone. This is from a shiur that I attended on the Rambam. and part of a discussion on the concept of "navi sheker"
If someone repeats the nevua without being commanded, he is a "false prophet".
The talmud in Bava Basrah 14b gives who wrote the actual books that the Anshei Kneses Hag'dolah compiled into the permanent books of the Navi that we have.
Something written on his own or for his own purpose is in Kesuvim as I state in What is the Difference Between Prophets and Writings?
an example of a source for this is Neusner, Jacob, The Talmud Law, Theology, Narrative: A Sourcebook. University Press of America, 2005