Not to downplay their achievements (after all, these countries get pretty decent results at Olympiads), but there is a statistically explainable trend in European countries having lots of people going to different Olympiads. When I discussed this with a certain famous Estonian IPhO person (people might know who I am talking about at this point), he pointed out that there are very few people (let alone smart people) who want to make it to any camp due to both population and Olympiad popularity reasons, which leads to a larger overlap.

As to how these people become this smart (I was more curious about how they get the time to study for this huge variety of subjects), a Swiss MO organizer (and IMO participant) told me that most schools there have a very relaxed schedule, so you're essentially free to do anything (but I believe this is true almost everywhere in the world), and they also have more exposure to Olympiads compared to other countries, apart from certain special university programs for school students like the ones that he attended.