I’m only speaking for Finland, but it’s probably similar for Sweden.
As for many other questions regarding the nature and geology of Finland, the answer is: ice age.
As a result of the ice age, Finland has a lot of lakes and those lakes are filled with islands. 55% of Finland’s islands are in the lakes (and we have 187,888 of those if “lake” is defined to be a body of standing water larger than 500 km2).
What happened is that while Finland was covered in ice (Weichselian glaciation began about 116,000 years ago and the last of the glaciers covering Finland melted about 10,500 years ago) the glacier moved as it melted. The melting water beneath the ice moved everything that could be moved and the rocks moving with ice and water ground the rock beneath (creating roche moutonnées for example). This created, among other things, valleys and elongated hills called drumlins. Ridges also formed at the edge of the glacier as the melting ice moved and pushed everything in its way. Later, what could be filled with water was filled with water, thus lakes.
Sadly, the Weichselian glaciation destroyed practically all evidence of what was in Finland before it: the plants, and the animal and possible human life.
If you look at south-eastern Finland (literally called “Lake Finland” region) on the map, you’ll see a lot of water. It’s difficult to see, but a large portion of this is actually just one (1) lake. Its name is Saimaa, and it is the largest lake in Finland. But it has so many islands and so many narrow straits that it’s pretty much impossible to tell just by looking at it. You can also tell how the ice age glaciers moved, just by looking at the map.
Here’s an arial photo from Lake Saimaa in winter with its many islands:
The rest of the Finnish islands are at sea.
As a result of the ice age and post-glacial rebound, the ground in coastal Finland is still rising. Therefore islands. Depending on what you count as an island, the Finnish Archipelago Sea region alone has an estimated 50,000 islands..!
Archipelago Sea east of Åland:
Some further island facts from Finland:
Islands make 3.8% of Finland’s total area
The official count of islands in Finland is 224,661
Only 10 of Finland’s islands are larger than 10 km2
785 islands are larger than 1 km2
75,818 islands are larger than 0.5 km2
549 permanently inhabited islands (excluding Åland region) have no road connection to mainland
Approximately 8,700 people live on those islands
Åland has about 6,500 skerries and islands, of which 60–80 are inhabited
Märket Island with its funny border has Åland’s only land border
Technically half of Southern Finland (including cities such as Helsinki, Turku, and Hämeenlinna) is the largest island in Finland as the area is surrounded by the Baltic Sea, Kymijoki River, Kokemäenjoki River, and Lake Päijänne
This argument was jokingly made because the largest island in Finland, Soisalo, is surrounded by two lakes (and rivers with rapids) with a 6-metre difference in water levels (generally speaking an island is surrounded by water that is equal in level)
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