That's an interesting point, and yes, it would make it easy if the game was only set in the woods.What if I cover myself with thick clothing? or bring an umbrella?you'll have to deal with the sun.
"Avoiding the sun" mechanic would lose its novelty pretty quickly; there was a mission in Mass Effect 2 where we had move quickly under the sun to not get cooked, there is a reason we only saw that mechanic only once in the whole trilogy + the 4th gaym.
But imagine you trying to go inside a town and the guard stops you for inspection, so you have to uncover yourself, exposing yourself to the sun.
It'd probably would give you a nasty penalty in general to persuasion as you'd look like some covered up hobo, probably merchants wouldn't even want to talk to you and tell you to go away.
Obviously that'd be hard to make a game like that, but there haven't been a lot of boundry pushing these days.