Often, they’re grading your papers on evenings and weekends because the conventional work day is already saturated with other obligations.
Professors know what it was like to be in college and write all kinds of papers.
But college papers aren’t written like letters; they’re written like articles for a hypothetical group of readers that you don’t actually know much about.
There’s a fundamental mismatch between the real-life audience and the form your writing takes. It helps to remember the key tenet of the university model: you’re a junior scholar joining the academic community.
No one is doing anything wrong in a situation like that.
It just means that further discussion of the assignment is in order.