During the 4-5 years of an undergraduate education, a student learns a lot of material, at least it seems that way to the student. But the truth is the entirety of undergraduate education is foundation knowledge. The piece of paper that says you have an BS or BA in field X is essentially certifying that you have the basic knowledge to function in field X. In a 4 year undergraduate program, you’re taught what’s already known by humanity, and what has probably been known for hundreds of years. Take my field of aerospace engineering. The technical courses in a typical aerospace engineering 4 year program include:
Calculus 1, 2, 3
Physics 1, 2
Design course 1, 2
I’m likely missing some courses, and I’ve left out the 2-4 lab courses. That’s a lot of different topics. However all of them teach material that we already understand, which is how they can be put in textbooks. So after 4 years in college, a student has just scratched the surface of their field and the how the world works as a whole. They have gained a broad foundation of the field, covering each subarea in a low level of detail, learning the fundamentals, but sufficient to get them started and provide a basis for future learning. Each subarea (aerodynamics, propulsion, structures, controls, etc) has it’s own massive body of knowledge that is never discussed in undergrad. There is just not enough time. That’s what graduate school or on the job learning is for. In graduate school, the student focuses on one subarea and gains more specialized knowledge. However these graduate courses are still taught from textbooks thus are teaching information that is already known. One step further is the Ph.D., where the student focuses on one topic of one subarea of one field in order to discover new knowledge. This is the goal of research. In research, the student is moving beyond what is known, and diving into what is unknown. There are no more textbooks, just papers written by fellow researchers also trying to discover new knowledge.
One of the things I miss most about graduate school was being surrounded by fellow lab mates who work in the same field and have similar research interests. We could discuss problems or technical topics. I could bounce ideas off them. We shared a common understanding and goal. In the wider world of a full time job, like being a professor, everyone has different specialties. So while all my colleagues at the university are intelligent and also hold Ph.D’s, they work in very different areas that don’t often cross with mine, so you lack the sounding board. And a sounding board is very useful when doing research where the right answer is not always obvious or intuitive.
So if you have an itch to learn, don’t stop at the undergraduate degree, keep learning for your entire life, whether through graduate school or by keeping your eyes open at work for new opportunities. I believe one of our greatest endeavors as an intelligent species is to understand this complex universe we inhabit.