Because busy work. If I make a year be 500 days long, I have to recalculate ages and make sure the players are aware of the difference, else people are wondering why that spry 28 eight year old is being talked about as if he should really consider refraining from hard physical labor all day long. Or trying to work out plant harvests to work perfectly with the new year, unless their life cycle gets changed, and oh screw it.

That's simply too much work for me to handle, and it is VERY hard to get the players to accept things like that in my experience. They just won't remember it, because it doesn't interest everyone and is difficult and doesn't have as much impact on the story as say, who the heck that NPC was who stole from them. Maybe someone with better organization can do it, but I think I'll rely on horned people running around to inform people that this might not be earth...