As a bit of an amateur photographer myself, I can explain at least some of this phenomenon.
Old film cameras, with manual focus, had a split-prism in the viewfinder that allowed one to nail focus pretty much every time, wherever you aimed the split prism. And you had to know how to focus because AF didn't exist. This is why you never saw many out of focus shots in those days.
Modern cameras, with advanced AF algorithms, lack these split prisms. They are often available aftermarket, but few photographers realize this. They rely on the camera. Without the split prism, you pretty much *have to* trust the camera; the viewfinder image is simply too small to be able to nail focus without that split prism. The AF algorithms are very good in general, but certainly not without fault. The real old, experienced hands can still nail it by using the distance scale on the lens; problem is, many modern lenses are going to new electronic focusing mechanisms that make the lenses "focus by wire", so the focus ring is not physically connected to anything inside the lens and will thus turn infinitely. As such, a distance scale engraved on such a focus ring would be worthless as tits on a bull, which brings us back to square one - having to trust the camera.
This will get better as more people adopt modern mirrorless cameras with touch screens that allow you to literally point on the screen to any place you want the camera to focus on, and it will do so. Or, in the case of some upcoming Canon models, when looking through the EVF, the camera will focus where you're looking. These cameras are still fairly novel, and thus expensive, so you'll have to give it time.