I’ve heard technical interviews are just as much about seeing how you approach a problem as they are about you knowing the answer beforehand. But apparently seeing what I do and don’t need to google is not included in that sentiment.
They're usually testing your problem solving skills rather than whether you've memorized a particular question - for example, how you approach the problem, how you break it down, etc. Having a good approach to problem solving in general is way more useful than memorizing common programming problems.