Yin Corpses possess extremely low levels of spiritual intelligence; even Flying Corpses, at best, have the intelligence of a five or six-year-old child.
Moreover, even Flying Corpses find sunlight distasteful, and while they are not harmed by it, they prefer not to be active during the day.
This particular Yin Corpse, on its journey thus far, had displayed a level of intelligence no different from ordinary humans and showed no fear of the blazing sun whatsoever—such behaviors were far from normal.
The most plausible explanation was that this Yin Corpse's cultivation far exceeded its apparent form.
When something is out of the ordinary, it usually spells trouble; the cultivators had unanimously abandoned their previous plans, choosing not to follow behind Chu Zheng anymore.
Yin Corpses often travel alone and lack the backing of Sects and powers. Presumably, they wouldn't possess much wealth, making it unworthy to take such a risk.