Because that's all they do.
When warm bodies are needed to stop the tanks, don't count on writers to be there with you. They'll be in their hotel rooms with their laptops and their notepads, writing. When you're busy making money the tried and tested way, yon writer will be busy wasting his time writing stories and filling his blogs: you're still saddled with the rent.
When someone is patently stealing your woman, writer, don't count on yourself showing up at their door and cracking skulls. You'll be at home, writing, adding one more neurosis to the ones you already have.
Cue the sound clip from American Sweethearts. Hank Azaria's Spanish character turns to a really barely-holding-in-his-psychotic-temper John Cusack and refers to him, derisively, as--
You'll also have the bonus of showing your unwary reader friend that the bedrock upon which he rests his sanity doesn't really exist. And then there'll be two of you f_cking up the world by making everyone uncomfortable with life as they know it. She was right who said it best:
Keep only cheerful friends; the grouches pull you down.
The world doesn't belong to contemplatives, besides. Writers in general never see the fruits of their labor. For every Stephen King and Neil Gaiman there are thousands of frustrated writers married to their own misery and (in my Mammon stories) at least one who is dating his misogyny.
You may be the next Nietzsche, the next Kafka, the next Rizal-- but look what happened to them.
Nietzsche: nuthouse, couple of strokes, death by tuberculosis.
Kafka: nuthouse, tuberculosis, death by starvation
Rizal: exile, death by firing squad
And if you luck out and do a Thoreau ... well, okay, he didn't suck. He lived a full life, though he was felled by tuberculosis at age 44.
The point is very few writers ever live to see their legacy; fewer writers ever get to have one.
What's greatness if you never get to see nor taste it? I'm altruistic enough to care about my fellow man, but I've read all the books and seen all the movies: writers end up with the girl and the happy ending only in the stories they write. And I'm sick of watching everyone else's happys ever after.