They're not. I get that you're asking a rhetorical question, but I used to teach university ethics classes, including a wonderful class called Radical Practical Ethics. Lots of educators, mentors, and usefully difficult people do indeed challenge us.
The question might better be phrased,
Why would any teacher use their position of power and authority to distract their students with artificially individualised moral riddles rather than analyse the structures and processes of collective oppression and inequality in play at this very moment - and how to dismantle them?
...which is another rhetorical question, I suppose, but it does get us closer to answering the original question.
When life gives you a platform for suffering beings, you can crave the audience and imagine you are helping, or work together to tear down the stage and the walls that keep all of you in the theatre.