This might be the explanation astralcodexten was discussing [1]. Geoffrey Miller, a commenter, (I'm not involved in EA enough to know if that's a household name) says this

    I've been to about 70 conferences in my academic career, and I'm noticed that the aesthetics, antiquity, and uniqueness of the venue can have a significant effect on the seriousness with which people take ideas and conversations, and the creativity of their thinking. And, of course, it's much easier to attract great talent and busy thinkers to attractive venues. Moreover, I suspect that meeting in a building constructed in 1480 might help promote long-termism and multi-century thinking.

Of course this man didn't buy the castle, so I don't think it's fair to hold a random commenter's thoughts against CEA. I'm not big on cognitive decoupling, but I also find buying a castle for a charity event venue shocking, and I don't think there were enough numbers in this post to soothe me. If renting event venues was too expensive, couldn't they have met virtually?

[1] https://forum.effectivealtruism.org/posts/xof7iFB3uh8Kc53bG/...