Most airlines see pilots as a "non-productive" employee. And lets face it, we are not the ones making money for the airline. If you dont work you make less money, because you are less productive.
An office employee gets paid a standard salary because he/she will stay after hours, and in many airlines not get paid for extra hours. So you get paid to work 9 hours a day, and actually work 12.. Now thats productivity. Dont get me wrong, I myself am a pilot and I wish we got paid standard salaries. It just doesnt add up for airlines.
And the other reason that they dont pay pilots fixed salary... because they can