The idea is that money/fame/"success" comes from writing novels, not short stories, so who would want to just write stories instead? Somehow there is the notion that one eventually "graduates" to writing novels and in the process becomes a serious writer.
That may be the view of the publishing industry, and it may be the conventional wisdom among writers that, currently, one can only truly make a living by writing novels. But it has long been understood by the writing community that the short story is not a dumbed-down, beginner version of the novel, but a completely different form. Even if one considers writing short stories as training for writing novels, mastery of the short form does not guarantee success with the long. Many MFA graduates, having gained considerable experience workshopping stories, find themselves at a loss when attempting to write their first novel - because it's not the same thing.
Good coaches put their athletes in the right events, based on their ratio of fast-twitch versus slow-twitch muscles. Not everyone is built to run a marathon; some are more suited to the 100-yard dash or the 440. And it may not have much to do with what a runner wants to do.
Of course, in writing physical limitation is not an issue; one can learn to write novels. But some writers feel they are built to work in one form more than another, and why should anyone write novels just because they think they have to?
As I've asserted previously, some writers write primarily for the art. Sure, we'd all like to make a living doing it, but commercial success is a secondary motivation. I want to write what I want to write, whether or not it gets me any financial gain.
Speaking of which, there are people who are known primarily as short story writers:
Tobias Wolff - my lifelong idol. He's written one short novel, Old School, and the brilliant memoir This Boy's Life, but otherwise, he has only (meant in the exclusive sense, not as a diminuitive term) story collections to his credit. If anyone compared me to him, I would consider myself undeniably successful.
Alice Munro - long known as a master of the short form, a staple in fiction syllabi. Highly recommended: her recent collection Too Much Happiness.
Amy Hempel - distinctive, spare, startling, deceptively intelligent writing.
William Trevor - while also a novelist, Wikipedia calls him "widely regarded as one of the greatest contemporary writers of short stories in the English language." Jaw-dropping and deep. 
Dave Housley - all right, this one is more of a personal prediction, and I don't know what his aspirations are, but his collection Ryan Seacrest is Famous is certainly an auspicious start. The stories in this book vary widely in characterization and circumstance, and are full of compassion without being sentimental. You'll see more of this guy, trust me. Writing short stories is not a consolation prize, a lesser skill I'm reluctantly settling for as my lot in life, a cute little hobby - it's a passion. Have some respect.
