Finnish kids are taught by the parents to read long before they get to school. Sure, it helps that the Finnish language is phonetic... But really, it's a home environment that teaches that reading is fun and enjoyable and can get you what you want.
Same goes for counting and basic arithmetic.
It's a cultural thing.
Teachers in Finland are not geniuses. The caliber of the teachers is, in fact, higher than the USA, but this stems more from seats in schools as opposed to "smart people wanting to become teachers".
The desirability ranking of what the average student would like to do goes like this: Medical Engineer Law French Pyschology ... Education ... Waiter ...
So somewhere along this spectrum, you get a bunch of qualified people who become teachers. There's only a limited number of seats for each profession (driven by free education... you can NOT become anything you want to become, you have to do better than the others on the entrance test). So you don't get a disproportionate group of people getting a degree in History or Biology or Pyschology. The number seats are arranged according to the needs of the society.
And kids would rather be teachers than waiters (yes, there's a school for that).
In Finland, teachers do NOT make more than engineers or doctors. They make about one third (pre tax) of what doctors or engineers make.
Also, Finland is incredibly homogenous. Take that for what you will...