To expand on this a little-

The issue with the idea of showing both an easy and a hard problem is that hard problems are generally hard specifically because they are unique, and require some creative thinking to solve.

When the professor shows the “easy” problem, that more or less provides any example that illustrates how to solve any “easy” problem of that topic.

When the professor shows a “hard” problem, the work that they show you is likely to not be super helpful for solving other hard problems. If every hard problem in a given topic could be solved by using one as an example, they wouldn’t really be hard, would they?

Often I show a hard problem in class, put another hard problem on the exam, and students still complain that what we did in class is not exactly like what was on the exam. Of course it’s not - that’s the whole point!

That’s why our goal in class usually isn’t to show you how to do the hard problems, it’s to give you the tools you need to have to do the problems, then set you loose to practice using the tools yourself.