Just want to understand, by normal do you mean less than 1/5 Finnish households? Going by the source below, I don't know if I'm missing something but it does not seem to be normal?
https://www.sciencedirect.com/science/article/abs/pii/S01692....
Many summer houses have shared ownership with a wider family. Say, your parents got one and now you own it with your siblings, each getting their own time there or agreeing when to visit together.
Even when you don't own one, you will very likely have participated in the summer house culture. It's very much part of the mainstream culture like sauna is. Maybe you visit your relatives' place, or you may opt to rent a place for a week.
Personally I feel renting is the best option as you can visit different places each year and avoid the maintenance burden -- and they obviously need active maintenance. But some do prefer a familiar place where they can spend all their holidays, and in some cases all weekends too on top.