The professor contacted 275 creative people. A third of them said “no.” Their reason was lack of time. A third said nothing. We can assume their reason for not even saying “no” was also lack of time and possibly lack of a secretary.
Every writer probably knows this already, but keeping a journal, diary, or just a notebook handy to jot down thoughts and impressions, pieces of dialogue overheard, etc., is an incredible tool. I found this old article from Psychology Today that states the case more profoundly in terms of maintaining creative flow.