Why don't people understand hypothetical examples?

We read W.K. Clifford's article on the ethics of belief in my philosophy class. In it he uses a hypothetical example of a shipowner who convinces himself that his ship is seaworthy by the use of clearly spurious reasoning (e.g. "it's never sunk before, so it won't sink this time.") Of course the ship sinks and everyone on board dies. Clifford concludes that the shipowner was wrong to believe that the ship was seaworthy. (Not just that the belief was wrong, but that it was wrong for the shipowner to have the belief because he didn't have sufficient evidence to support it.) Furthermore he argues that the shipowner would have been wrong for believing the ship was seaworthy even if it had not sunk. Clifford's thesis is "it is wrong, always, everywhere and for anyone to believe anything upon insufficient evidence."

Now there are many plausible criticisms of Clifford's argument, I don't want to suggest that it is unassalable. But I'm amazed at the number of times students will say things like "how do we know that everyone died? Maybe someone survived." or "maybe the ship didn't sink because it was in bad shape, maybe it sank because of a storm." Of course one problem with such response is that they just aren't relevant. Clifford's point is about evidence, not about ships that sink. But my puzzlement is in a different area. It's a hypothetical argument! Clifford is making up an example to illustrate a point. He gets to decide how the example goes. You can't argue against it by questioning whether Clifford has got the details right. Of course he has (or hasn't). It's a hypothetical argument! How can people not get this? Did they not do story problems in math? Imagine a student raising their had in 7th grade math. "Excuse me teacher, how do we know the train was really heading east from Chicago? I mean what if it went south for a little bit and then kind of northeastish?" or "You shouldn't count my answer wrong because after all, we can't be sure that Sally really had six apples before she gave three to Bob. I mean who saw her? What if it had been seven and she just miscounted?"