Student assignments also rarely operate in the sheer magnitude of real world programs.

In university, I think the longest assignment I had was in the ballpark of a couple thousand lines of code and you could pretty readily keep it all your head as you solved the problem, there's not too much abstraction or delegation of responsibilities, no interfacing with 3rd parties, few library dependencies, it's all written in a single language, etc, etc. And you wrote it all from scratch by yourself or in a small team with (likely) equally inexperienced peers, so you know all the clever tricks that were employed.

On the job, I deal with company's code base that's over a million lines of code and uses a very messy database, maintained by dozens of people or varying skill levels over the last decade, wrought from an unholy amalgamation of 3 different programming languages spread over several servers and reliant on a half dozen "support utilities" to keep applying band-aids to bugs that "aren't cost effective to fix".

Writing good code the interacts with bad code is one of my hardest professional endeavors.