There are whole cultures living in swamps in the real world (e.g. in southern Iraq)...
However, it's true sensible people wouldn't build a house to live in in a mosquito and/or gnat infested Finnish swamp. However, there are other reasons to build structures in swamps, such as for hunting or storage of items used in the vicinity.
Also remember that the game doesn't force you to do things you find unreasonable: if you don't think it makes sense, just don't do it. UrW isn't the kind of game where "it is OK" to exploit every quirk you can find, but rather one where you play as you please (it's a sandbox single player game, after all), and many players use house rules to limit what they do. Production of planks for sale would allow you to buy out everything in a village in a fairly short time, for instance, and most people don't find that enjoyable, so they just don't do that.