You say careful, and their ego probably hears you calling them careless. Careless is typically an avoided behavior so this hurts them. They view themselves as careful and wise. You think they're dumb. (Not necessarily a syllogism, but the ego is weird)

I'm possibly paraphrasing and simplifying too much but "How to Win Friends and influence People" mentions being very cautious about correcting people, more so correcting them publically, even when you are correct.