There are just some things you'll never learn, for example, until you have to interact daily with code you wrote a year ago.
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.
PS: At best a comment only says what someone thinks the code does. Program for long enough and you will get bitten by this.
Bottom line: in my experience, the reason most programmers are self-taught with regard to writing good code is not that no one tries to teach them. It's that most of them don't think they NEED to know (i.e. don't place enough importance on it) and are willing to lose marks for it until they get to a job and they have no to choice but to, you guessed it, teach themselves. There are people who try to make the best out of their education, and these can be very good coders who learned by making the best of their education.
(Note: obviously this is a generalization - there are plenty of students who try to write good code.)