They hide them so the crooks all know where they are and regular guys do not!
In 1987, the Department of Transportation's Motor Vehicle Theft Prevention Standard
required manufacturers to also put the VIN on the major parts (like engines, hoods and fenders) of certain vehicles if the car is considered "high theft" [ref
On most cars, you can find the VIN on the dashboard on the driver's side, and it's visible through the windshield from outside the car. It is usually on a sticker or plate on the inside of the driver's side door or on the frame sill where the door closes. The VIN is sometimes printed inside the glove compartment. I believe most have a vin code stamped into the floor under the driver seat too.