Because a huge proportion of the time there's nothing useful that you can actually do when you catch an exception, other than print the stack trace and exit. And because people are forced to catch the exception instead of letting it bubble up and stop the program, half the time they just swallow it and ignore it, which is even worse.

A decent roundup of some of the arguments for and against: http://www.ibm.com/developerworks/java/library/j-jtp05254/in...