Actually there are really a lot of such stories. Almost every newspaper, magazine, webpage or just plain blog has them quite often.
Well, reason is really simple. We like to read them. A story of overnight success is a proof, that everybody can achieve his/hers part of success. And as those stories are written, such an overnight success is achievable literally overnight.
Whatever the goal of a person who achieved such a success is, everything is possible. Losing weight overnight, falling in love with the most beautiful woman overnight, landing a dream job overnight, earning millions of dollars overnight.