Why don't car owner's manuals specify all the locations where it is safe to jack up and support your car?

They usually only show you where to put the trunk jack to change a spare. The factory SERVICE manual likely has all of the other approved locations. And it has a whole boatload of other info that's important for servicing the vehicle yourself, like torque specs and what bolts are throw away. But the easiest way to jack up yourself is to get some pinch weld adapters/pucks and use them with your jack or jackstands at the pinch weld locations. A shop would likely use these with a lift. Otherwise, my Service manual showed me additional center points, front and rear, that I can jack my car up with. So I do that with a jack and then use 4 jackstands with punch weld adapters at the 4 pinch weld spots. They do that because they really don't want you to service the vehicle. The owners manual normally doesn't even explain how to change the oil. Often it only has how to measure and top off the oil.