So a $80 million yatch can't afford an $3000 high quality gyroscope?
Now you could buy amazing laser gyroscopes, for planes the Inertial navigation system error could be great, but for ships(that move more than 20X slower) is not.
Humans, like water, take the easiest route. Unless someone is particularly vigilant and routinely checks the gyroscope to verify the course the GPS is using you wouldn't notice the "hijack" till it was too late. Most people I know wouldn't be paranoid enough to verify their own course against a gyroscope, or any other means of determining coordinates.
True, it could be. If they went to the trouble to integrate the two and develop the solution. But that begs the question: why use GPS? Use the same map information with the non-GPS location calculations that could be automated. GPS is just too convenient...
GPS is convenient, but it has other advantages as well. It's almost always available, it's highly accurate, has a quick time-to-fix, etc.
Inertial navigation systems require finicky setup and drift over time. (Ask the passengers of KAL 007 about that one.) However, the setup can easily be automated using GPS inputs, and the drift can also be corrected that way. This way, you have two systems which help keep tabs on each other. The INS can't be spoofed, and so can tell you when your GPS goes out of whack. The GPS won't drift, and so can keep your INS up to date when it's not being spoofed. Cost aside, two systems are better than one.
Another interesting possibility would be to use a Star Tracker[1][2] for absolute fixes to sanity check GPS / recalibrate the INS periodically. And it has a long and storied naval tradition!
Now you could buy amazing laser gyroscopes, for planes the Inertial navigation system error could be great, but for ships(that move more than 20X slower) is not.