I’ve never read and rarely seen a book-length interview with someone or transcribed lecture in English, but in Japan it seems like about 30% of the non-fiction section are exactly those two things. I’d long assumed it was just because of the “churn them out fast and cheap” philosopy of Japanese publishers, similar to the reason why reality television has taken over British TV. Still think that’s true, but there may well be another reason.
According to a student of mine and what I’ve read elsewhere, spoken and written Japanese are quite different from each other, more like spoken and written German or Greek than the comparatively similar spoken and written English. A consequence of this that I hadn’t thought about is many Japanese people going for books which are as close to spoken English as they can get, hence interviews and lectures. Probably even more true amongst the tired commuters who are a main market.
Might also help explain the popularity of comics amongst adults, of course.