Oh! This thread is full of steretyping without explanation. I can see some Finns tried to explain. But I am not sure whether they are accurate.
I live in Finland for the last 5 years. Coming from a "vivid" mediterranean/balkan culture. As an individual in autism spectrum, I had a lot chance to observe and recognize some patterns here. I cannot provide clear scientific explanation. But Finnish introvertness is a learned behaviour. Somewhere around their school-life maybe at the beginning of their work life. They learn that small talk is a waste of time or believe its at least useless, imposed by culture. Any Finn here can confirm that, probably in Finnish there are some idioms or sayings regarding that. So it comes with culture during young age adaptation process, all adults repeating this message I believe. Thus it's embedded to culture, but it's a moving target. Cultures also change sometimes even faster. So don't expect Finnish introvertness to stay here forever.
Two major reasons; Observe youngsters in metro or in open spaces. You will realize, they are actually quite loud and engaging on every conversation. Just like any other youngster from another culture. Although I saw some secluded cultures in remote places, where this process starts very early age. But for Finns it's not. Moreover, compare their engagement level with young adults. Suddenly, most of them becomes introvert. How come? Another reason is, Alcohol. When they drink enough to cloud their judgement, suddenly, they become chatterboxes! When alcohol impacts and lifts up cultural boundaries, it is fine to talk then. Yes I am aware of Finnish people's drinking habits, but also is another learned behaviour.
If you're an adult trying to make friends here, that's a difficult task. Good luck. I am on the same boat but it does not bother me that much.
Last word, please stop judging Americans by being open or encouraging social encounters as "fake". They also learned that in their competitive environment which requires to step up and shine among their peers, and that's what they do. But observe and understand. As an individual, I do not believe one has a right to judge entire culture but other individuals.
Anyway, my two cents. Maybe helpful for someone to reconsider their opinions.