A friend of mine flies CH-47 Chinook helicopters in the U.S. Army. He was asked how without keys, what prevents someone from just climbing in, starting the helicopter and flying away with it. He responded: “By having an extremely complicated starting procedure.”
For large complex aircraft, especially those with turbine or jet engines, the starting procedures are pretty daunting. In addition, unless you’re type-rated on that particular model aircraft, the starting procedure can vary from one model to another.
Even with flight sim practice, it can be extremely problematic. The vast majority of people will not be able to do it…