The logic between GS and side hustle all comes down to the objective. If you are going after immediately spendable cash, the GS generally wins. It requires the skill set that we have developed over years of education and experience and commands a high price per hour.
Sometimes pilots just want to exercise an different set of skills and chose something else they may want to carry with them into retirement to stay mentally sharp.

If you have a longer time horizon than a few months a profitable side hustle can make a lot more sense, especially if you are developing some form of residual income. A day or two spent working on real estate investments for example won't make you much money that first month or even that first year, but on a consistent basis over time, it builds up like rolling a snowball down a hill. With the right approach after a 5-10 years, flying airplanes can become your "side hustle".