Others here have already covered some of the main reasons a lot of people write on different sites and/or blogs. The eggs-in-a-basket thing is one of the more important reasons (in my opinion) for someone to be very careful if they're writing articles on a site (as opposed to writing on their own site or blog); because so much of what goes on with/to someone else's site is beyond the control of the writer.
Separate from all those already-mentioned reasons: Particularly on this site, it's not all that easy to manage one account with too many Hubs in it. I've written 400-500 Hubs, left around 330 most of the time, and recently trimmed out almost half; and it's still time-consuming and thought-requiring to manage as few as a 130 (or so). It's a whole lot simpler and easier to manage one's own site and blog, especially if one has chosen not to add all kinds of images and other stuff to his posts.
Separate from even that, though, there are people (I'm one) who are just the type to write and/or who have their own agenda about what would be a good thing to have online (or else, even what's only something they want to have online, whether it's "a good thing" or just something they enjoy or want or need to do. I have all kinds of "foundations" for blogs (undeveloped blogs with plans) or sites I've set up for one reason or another (sometimes to organize, sometimes to write I feel like writing, whatever....). I see online writing as something that's here to stay, and I make choices about what I do with my writing or ideas based on long-term plans/thinking/possibilities rather than necessarily on what gets immediate traffic today. What gets traffic on the Internet is ever-changing and subject to ever-advancing technology; so I operate both within what might work today and what I may one day be glad that I got started today (and continued to build on) over time. I also aim to lean most to efforts that I think will be a little more resistant to the "whims" of the ever-changing Internet. I'm not above sacrificing short-term traffic over my own varied, long-term, aims as a writer and as a person.
Oh... and then, too, there's also the "niche factor" in a lot of cases. :)