In Introduction to Computing Theory (senior undergrad class — why do they always label the hardest topics “Introduction to”?), we learned that there are a “countably infinite” number of integers (or natural numbers). You can enumerate them forever, without getting to the end.
However, if you consider real numbers, you can enumerate forever and never even get from 0 to 1. Therefore, this infinite is infinitely more infinite (or something, I never really did get that).
They did the same thing in graduate applied physics. Introduction to solid state physics, intro to accelerator physics, etc. Because the best they can do is introduce you to it. The more you know, the more you realize that you cannot possibly know it all. The sum of knowledge is too vast on any particular subject. So you learn what you can, specialize in what interests you and hopefully you can make a contribution to further that sum.