Link: http://www.usatoday.com/money/biztravel/2005-05-04-airline-troubles_x.htm?csp=34

This article from earlier this year in USA Today does a reasonable job of explaining how most of the major US airlines have managed to stay in business, despite many of them having been in Chapter 11 at least once in the last few years (and in some cases, more than once).
The article shows that, while the barriers to entry into the airline business are formidable, the barriers to exit are even more daunting...any airline that tries to liquidate itself is facing an almost impossible task, which may help to explain why several unprofitable US airlines continue to hang around.