Aircraft software (airborne software) is documented to death, written in one of a few certified compilers, walked through and tested to death. It runs on old, very well understood processors and is generally pretty simple - look up tables with simple interpolation algorithms. All the data is developed on the ground, slowly, carefully and under a microscope. There are more than one of everything in the plane and if they disagree, they shut down and the pilot takes over. Yes the results are clever, but the implementation is clear. It is written for one type of aircraft at a time. AND IT IS VERY VERY EXPENSIVE!!!!!
Compare to the above - state of the art hardware (Pentium FPU anyone...), Consumer O/S (enough said), commercial constraints and minimal regulation, Dozens of types and models of cars, brakes, engines, steering etc etc.
Its a bit like Mainframe vs PC - would you trust your life to a PC?