Well, for one thing because I'm a big believer in Paying Forward. I'm humble enough to admit that I wouldn't be anywhere in this industry if not for the wisdom more experienced authors were generous enough to share with me when I was first starting out. The best way to pay homage to those great writers is to emulate their example.
Another reason? (Warning: a little more mercenary) Readers are more likely to buy a book from someone they feel they "know." In this harsh economy, people aren't as willing to part with hard-earned cash for an unknown. And if you don't show up at speaking engagements, book events, get-togethers with other writers, and other social opportunities because you're "too busy," readers won't get the chance to know you at all. The more often you participate in various appearances and events, the more often you volunteer your time to others, the more readers you meet and the more readers will get to know you.
Need another reason? How about it's just plain rude and egotistical? Yeah...I thought that one would reach you.
For tips on writing and fun articles, visit Gina's Articles For Writers page: http://www.ginaardito.com/ArticlesforWriters.html