It's not that you shouldn't cite them, it's that you shouldn't use them as a source at all because they're considered unreliable for the subject you're working on.

Depending on the point you've reached in your learning career, you might not be equipped to detect and criticize an outdated source.

Some fields also evolve so quickly that what was considered a fact just 20 years ago might have been superseded 5 years later and again 5 years later so the only info that's considered reliable is about 10 years old and everything else must be ignored unless you're working on a review of the evolution of knowledge in that field.