1. Fuel is liquid currency. Consider that you're leaving the equivalent of a bundled stack of banknotes on the wing.
2. Fuel, particularly avgas, burns very admirably in motor car, bike, quad bike engines, or any other petrol-fuelled engine. Nearly everyone needs and uses fuel.
3. Thieves regard nothing as sacrosanct. If you can drive into an airfield without regular or covert scrutiny, so can thieves.
4. If you understand all of the above, then there should be no reason why you wouldn't take all the necessary steps to ensure your fuel security to the maximum level possible.
5. Anyone who takes off without physically dipping tanks is guilty of poor airmanship, and guilty of developing a dangerous habit.
There was a major event involving an Aero Commander 500S that took off from YPJT on 27th February 1986. The PIC had fuelled the aircraft the previous day, and there was no aircraft activity in the log since he had refuelled.
Unbeknowns to him, though, extensive ground testing of aerial survey equipment fitted to the aircraft, incurred substantial time running the engines.
The PIC took off without dipping the tanks, despite also knowing that the fuel gauges were inoperative, and ran out of fuel within 90 minutes, in heavily timbered country in the Darling Ranges above Perth. Both occupants suffered fatal injuries in the crash.
Investigation: 198600131 - Aero Commander 500S, 2km North of Canning Dam WA, 27 February 1986