Google changed that, to a degree, a few years ago. Today they can pull out a keyword from the URL even if it's only separated by an underscore. However, the value of any keyword in the file-path is quite negligible, so it's not worth the many challenges of changing existing URLs.
So the way Google handles underscores is better today, but dashes are still smoother sailing. Reference: Dashes versus Underscores [mattcutts.com]