Yes, the reason is so I could be paid for my writing. You can see my reasons for not being a member in other comments on this post.
It could be that you want to connect with like-minded folks, but that’s not a motivation for me. I’d rather connect with the author of a book because when you’re reading a book, you’re connecting with the best possible version of the person who wrote it. Internet comments on the other hand—well, that kind of thing is like a needle in a haystack.