Would Binx even want to return to a human form at that point?
Max and Allison look through the book to see if they can find a spell that could change Binx back into his human self. But if you were in Binx's position, would you prefer to remain a cat? Of course at the end of the day, Binx's goal and desire was to be reunited with his sister Emily in the afterlife.
But let's say that Max and Allison actually manage to help Binx turn back into a human. How is Binx supposed to adjust after spending the last 300 years as a cat? I guess I would equate it to how people who have been incarcerated for a long time contemplate trying to get back into prison because they feel so lost and unadjusted to life on the outside.
I mean, Thackery Binx as a human in 1993 has no family to go home to.