Why isn't luggage that goes in the hold scanned as thoroughly as hand luggage?
The main reason why checked luggage is treated differently is that you cannot access it during the flight, therefore eliminating some of the security concerns (like e.g. a knife in checked baggage is not a risk). When your carry-on bag is hand-searched, it is most likely because something could not be seen properly or looked suspicious on the X-ray.
The checked baggage is still scanned though. The TSA writes:

TSA screens approximately 1.4 million checked bags for explosives and other dangerous items daily. Upon check in, your checked baggage will be provided to TSA for security screening. Once the screening process has completed, your airline will transport your checked baggage on your respective flight as well as deliver it to the baggage claim area. The majority of checked baggage is screened without the need for a physical bag search.
Inspection Notices: TSA may inspect your checked baggage during the screening process. If your property is physically inspected, TSA will place a notice of baggage inspection inside your bag. This is to inform you that an officer conducted an inspection of your property.

Note that the TSA will notify you with a notice in the bag when they opened it. Other countries might handle this differently and just open the bag without telling you.
Some more details of the screening process are given in this article (emphasis mine):

Soon after your suitcase vanishes behind the rubber curtain, it merges with the bags of other passengers -- in large airports, thousands an hour. After traveling up, down and around a vast network of high-speed conveyor belts, it arrives in a cavernous warehouse-like facility. Here, it will be scanned by Transportation Security Administration staff to make sure there's nothing in it that shouldn't be there, primarily explosives, illegal drugs or weapons. If they see any reason to open it, they will. Your bag might also be subjected to other checks, including sniffer dogs, X-ray and laser scanners and machinery designed to detect trace amounts of banned substances. If it passes muster, luggage handlers load your suitcase onto a wagon with the baggage of others on your flight. This wagon is driven onto the tarmac and its contents packed into the hold of the plane.