at the risk of stating the obvious here, why the hell should you after studying and working all your life to get an airline job in the first place, then have to do the same thing on your days off just to make ends meet...hello!
BTW show me one person you know who ever got rich, or even made a living from day trading, whether it's trading shares, currencies, CFDs or whatever. The reality is you need to watch the markets 24/7, and if you know how you can do that when you are at 40,000ft then you are too smart to be a pilot in the first place.
And I am sure the 300 passengers behind you would love the fact that you are thinking about your open position in pork belly futures rather than doing what you are being paid to do, ie fly the plane