That’s because there is an entire laundry list of things that can cause such a problem. It isn’t just one thing.
In most of the cases that I’ve dealt with, the problem has been due to a minor software error that gets logged over and over and over, until the relevant log file balloons to the size of all of the free space on your drive.
See:
https://web.archive.org/web/20170321110308/http://Pondini.org/OSX/DiskSpace.html
for the mother of all suggestions on where your hard drive space may have gone.
I’ve also seen Dropbox and TimeMachine cause this problem.