Pretty sure the island was populated when the machines attacked so there should be more bodies around.
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Or at leastmake piles of bodies like in left 4 dead, those are not lootable but add a lot to the atmosphere of the game.
Maybe someone hauled the bodies off to somewhere for sinister purposes?
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Im not sure but i have a general theory…
Spoiler
The machine/brain-interface is mentioned in the storyline, so its very possible that the initial battle was focused on capturing people alive to be used by the machine-“collective” as “pilots”.
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You learn early on that most of the people were evacuated. Those few who remained behind, those you’ll find in the world, alongside military personnel who fought the machines.
There is another reason why there may be few people
SPOILER
In the final mission “Behind the Curtain” There are many humans that are hooked up to these strange devices on the monitors perhaps this is where most people are? 
At first i thought the lack of bodies was because the machines needed a form of fuel. “Biofuel” and used the bodies of humans to make it lol
Actually they added a lot of dead bodies, each region they “upgraded” gets a lot of new dead bodies, so i believe when they finish upgrading Mountain, Marshlands, Forest and North regions, those Regions will be as Himjäll, Archipelago and South coast are now (dead bodies everywhere 
).