I looked into this and it isn't true - countries which switch to opt-out don't get significantly more than we do. I agree this is surprising. The best explanation I was able to find was that, of the few people who get in accidents/comas where their organs are useful, most have already opted it, and there's a lot of negotiation with the family around exactly what that means such that an opt-in/opt-out box doesn't make a big deal. See eg https://www.hopkinsmedicine.org/news/media/releases/presumed... .