science

The number is so low because schizophrenia is super rare and blindness is also super rare. Add to that that blindness is much more common in developing countries where the diagnostic capabilities for schizophrenia are basically inexistent.

Just multiplying the odds to have schizophrenia with the odds of total blindness from birth gives you incredibly low chances to have both at once.

With such low numbers random chance is a huge factor, so it's quite likely to not find anyone who has both.

You could do the same with any two other super rare conditions and you'll have a high chance for similar results.

For example, you might be hard-pressed to find someone with an IQ under 30 and schizophrenia.