Most people are price sensitive and want to be taught to the FAA minimum standard. And considering that there’s three levels of minimum standards... nobody is going to know everything at the Private level. Heck, nobody knows everything at the ATP level.
Many describe the Private as “a license to start learning”. For better or worse, 40-Ish plus hours in the air and basic ground school isn’t anything but the tip of the iceberg.
And if an instructor can convey that fact as well as an ability and curiosity and motivation to learn autonomously in the student, after they’re long gone, that’s pretty decent.
FAA has levels because it would be difficult to justify making everyone understand “Aerodynamics for Naval Aviators” and test them on the math before calling them a pilot, for example.
As long as one knows they met a minimum standard and doesn’t accept staying at minimum standards as a pilot, there’s always something new to learn or practice.