Why is Thieves Landing perpetually dark?

I never understood this design choice. On even a sunny day, the sun will completely change to dusk the moment the weeping willow trees begin. Is this supposed to represent a never ending overcast sky over the swamp? Are bayous known for never having clear skies? Or is it simply a way for the game to fake a density of swampland foliage making the area feel denser than it geometrically is?

I really hope this isn’t a continued design choice for the swampland areas in the new game as it always finds a way to take me out of the original game.