The real reason to write novels
Because the business side of short stories is a pain in the butt!
I've got a novel I'm working on (one of many) which grew out of what I thought was going to be a short story idea. I like the novel, but I felt there was still potential for the idea as a short story. So I started over with the short story part of it. It came out at about 3400 words. Of course, I need to go back and revise, and send it out for critique, and revise again before I even think about submitting it anywhere.
But once the revisions are done, then the hard part starts... the submission process. It's bad enough submitting novels, but submitting short stories is just a pain. Half the places don't take e-submissions (I can sympathize with that, e-submissions are simply too easy to do, resulting in a billion bad stories flooding your inbox). So you've got to mail off your story, wait two to three months for an answer, and then, if you're rejected, which is a good possiblity, start it all over again. Figure five or six rejections (at least) for one story before you sell it, and you're talking a year and a half or more to make a sale, since no one wants simultaneous submissions either (again I can sympathize, but gosh for any author trying to make a living at this, it is a real pain in the neck).
Of course, you also have to figure in hours of researching good markets for that particular story, and the cost of mailing it out repeatedly (and don't forget the cost of the SASE).
If you're trying to make even a modest income from short stories, you've got to publish a LOT of stories -- 1 or 2 a month. Tracking all those submissions is a nightmare in itself. It's bad enough when you get a response, but some magazines don't bother to send you a polite "no thanks" even though you sent the SASE, so you are stuck guessing, should I send this off yet.
Having been doing this for a while, I lean towards the give them three months and move on strategy.
So, why bother with short stories. Cause common wisdom is that agents and publishers like to see that you've been published before. It gives them a sense of confidence in your writing ability. In it's marketability. Of course, that should be obvious from the writing itself, but if it's going to give even a tiny edge to a novel's chances with an editor or an agent, then guess I'll continue jumping through the hoops.