Small planes (like the little Cessna in How Airplanes Work) have locks on the doors and ignition keys inside to start the engine. They are pretty much like a car in this respect. The reason why they need locks and keys is because they sit out in relatively unsecure parking areas sometimes for weeks at a time. They need locks for the same reason that a car does when it is sitting in a parking lot.

Commercial jets, on the other hand, have no locks on the doors and no ignition key of any sort. You can hop in, flip a couple of switches and start one up!

The reason why jumbo jets don't need an ignition key is the same reason that you probably do not lock your car when it is sitting in your locked garage. Since the garage is secure, there is no need to secure your car. Security at an airport is generally very good, and planes are kept locked in hangars or attached to jetways that are secure and under constant surveillance. In addition, if you did somehow make your way into the jet and start it up, the chance of your being shot down or caught is 100%. That provides a good deterrent.

Nonetheless, people do abscond with jets. That is what a hijacking is all about. The fact is that hijacking is remarkably rare because of all of the security and the deterrents already in place.