1. Because I am very organized.
2. Because it makes me feel mildly altruistic even though it actually means nothing.
3. Because it creates slight difficulties for someone who would dare to create a non-profit or foundation in my name. I do not know why anyone would do this at present, but better safe than sorry.
4. Because (related to the previous item) in the event of my untimely demise or sudden fame, my estate or my team (depending on the scenario), will have a head start on securing a good domain name for the charitable organization that will inevitably be founded in my name/memory (again, depending on the scenario).
5. Because it affords me the opportunity to point out the common misconceptions that .org sites are exclusively for non-profits. That is factually not true. They were simply originally intended for non-profits but anyone can use them. It's like ordering a Happy Meal at McDonalds as an adult. They can give you a funny look, but they can never, ever stop you.