Because, consoles.
Bethesda couldn't make the game too pretty because of the outdated console hardware, and the UI had to be dumbed down enough for the smallest of kids getting Skyrim for their xbox this christmas.
Stupid pc kids, wanting skyrim to be "pretty" even though the graphics are one of the most improved features of the game certainly good enough to modern standard with nice textures and smooth well modeled faces although it could have done with 50% less dirt, but it's the last thing the game needed more time spent on replacing textures all over the world, making more complex and detailed models and wasting thier time on things that will get replaced in a year max. Smallest of kids? Other then you TES isn't a game your typical 4 year old will play, congrats on getting that awesome pc rig from Santa but for stop crying that the game isn't pushing all of its boundries, that is a completely unrealistic demand and only results is Duke Nukem syndrome or worse. Maybe the better light source physics isn't as ridiculous a demand but then again how much would that push save files and processing of these millions of light sources. I'm sure someone will or even maybe already has done a better light source mod but likely only 5% of the player base will be capable and want to use it anyway without any issues.
Of course. I heard the current economic climate is due to console users too.
Said it so much better and funner then I did.
As for the fact mods made already are proof of consoles ruining your version by making it a poorly ported game, I wouldn't be so sure the same modders with access to the same resources couldn't polish up the xbox version and as you even stated UI is a personal perference so the fact a mod of it exists regardless of length of time after the games release is proof of nothing. The xbox version isn't all people seem to think it is, aside from potential better textures, faster loading times and other things a pc can do which consoles can't, you have access to the in game console to fix the game glitches that console users get like ones resulting from a broken questing system (not being able to recognise you have an item in your inventory beyond the dragonstone etc.).
It's amazing how many topics become either a console slagging match or "skill" endowment contest to show only certain people should have the right to play a game, how dare anyone not born 20 years ago get into the series, I bet I pee off those people by playing when I have a nerve problem in my hands so not the best of reflexes.
If you really want dungeons to be so dark turn off your TV, that boost the risk factor greatly ha.
Completely unrealistic but I'd love an option to play the skyrim main theme from the systems menu as I walk around, It's just so epic it's a shame you hear it so rarely.