Part of the purpose of training for a doctorate (and one of the things that they will test for in a final viva examination) is to demonstrate the responsibility

Part of the purpose of training for a doctorate (and one of the things that they will test for in a final viva examination) is to demonstrate the responsibility to not claim something to be true when you don't have sufficient evidence. So therefore, people with doctorates should typically say "I don't know" more often than other people.