Because I'm a writer.
You see, writers (and actors too, for that matter) are creatures of the dark. We stay indoors, sit in our writing space, away from the distractions of the world (like the sun), and we write. Our pasty flesh often rivals that of professional gamers due to the number of hours in front of a computer. Some would consider that a bad thing, but it's not. It's a badge of honor that says "I'm dedicated."
I know, it is true that I could write outside. My laptop is very portable, but the screen is easier to read in a dimly lit room. Yes, I could go old school and write by hand or do it the Kevin J. Anderson way by using a hand-held recorder, but then I'd just have to type it up later. I may as well save time by typing it from the start...
You see how this goes?
There are very few arguments for this that can't be countered with "but it's easier to do inside." It's a vicious cycle, so why fight it? One word: SUNSHINE!
You see, like all human beings, writers need sunshine to thrive (and to keep us from slitting our wrists). Doctors are still arguing what those of us in the Pacific Northwest already know to be gospel truth. A lack of sunlight makes you depressed. I'm convinced that Edgar Allan Poe would have lived a long productive life if he'd laid off the booze and gotten more sunlight.
The answer to this is to pace yourself. Write for a few hours and then take a break. Go outside, do some gardening, mow the lawn, walk the cat. Soak up some precious rays before you return to pounding the keys. You'll be glad you did.