Journalists chase the story because that's who they are. The better the stories, the better the portfolio. The better the portfolio, the higher you climb the money ladder. If you don't climb the money ladder you don't make squat. For example, I graduated in 1994 with my journalism degree. My first newspaper job paid a salary of $14,000 a year. If I took my weekly paycheck and divided it by the number of hours worked, I wouldn't have made minimum wage. In contrast, my roommate who graduated with an elementary teaching degree had a starting salary of $24,000.
Most stories in newspapers these days are Associated Press stories. For example, if you look at many of today's front pages, they will have coverage from Baltimore. On my front page two writers are credited: one is a long standing AP reporter and CNN correspondent, the other is a recent college graduate who has been working for the AP since last fall. The recent grad won't make any extra money for her co-authored story that will likely appear on the front of numerous papers (hell, she might be an unpaid intern at this point). But she will forever have it in her portfolio and will use it to climb the ladder.
Of course, Baltimore papers aren't using AP since they have their own reporters. Editors assign reporters to cover different angles of the story.
Nowadays, there are many who can't find or don't want traditional jobs and go out on their own in the hopes of finding a big news story and selling it to a publication or news network. Richard Engel and Anderson Cooper are great examples of this. Both men though came from money and had the financial means to freelance before they hit it big.