The one thing to keep in mind with a modern airplane is how much information is available in the cockpit. If you leave the gas cap off on your car the only way you'll likely find out is if the 'check engine' light comes on twenty miles later. Tire pressure's likely unavailable and your oil pressure's probably just indicated with an 'idiot light.' Contrast this with a plane where data on almost everything is available in the cockpit. So the 'walkaround' is just to make sure the engines haven't fallen off, there aren't any obvious problems etc.

Cheers,
Geoff Glave
Vancouver, Canada