Dirtbag: A person who is committed to a given (usually extreme) lifestyle to the point of abandoning employment and other societal norms in order to pursue said lifestyle.
Ref: Urban Dictionary
Why dirtbag pilot?
Flying a small plane is fun. Modern flight schools are determined to strip every bit of enjoyment out of flight training to maximize profits to the point of making it a brutal grind for the students and then an even more brutal grind once those students become instructors themselves.
Dirtbagpilot takes a plane of the same variety you’d see at a flight school and uses it to an incredibly good time.