Earlier quoted context omitted.
I think they just enjoy memorizing things. Roughly the equivalent of meeting someone who runs 10 miles a day. They enjoy it and it has some benefit to their life as well, even though they are probably far past the point of diminishing returns.
More precisely, enjoys memorizing things but isn't good at it. Professonal quizzers can absorb huge amounts of information but typically don't use flashcards. OP is grinding thousands of cards a day to get to the same level - worthy of respect!
Professional quizzers almost all use flashcards. Jeopardy! people definitely do. The range of questions that you can ask in a general knowledge quiz are surprisingly limited. You're not going beyond a surface-level overview on anything. Do a ton of general knowledge flashcards for a year and you'll annihilate people at pub quizzes.
I suppose you could just read encyclopedias over and over again, and books of lists.