Why is re-reading a bad way to study?
Because it produces a feeling of knowing that is not the same as knowing. This is sometimes called the fluency illusion, and it is the reason re-reading remains the most popular study method despite consistently performing poorly in comparison studies.
When you read a passage a second time it feels familiar and easy. Your brain interprets that ease as competence. But familiarity with the shape of the text is a different thing from being able to produce the content when the text is absent, which is what an exam requires.
Highlighting suffers from the same problem and adds a second one: it feels like engagement while being a largely passive activity, and heavily highlighted pages tend to mean everything was marked important, which is equivalent to nothing being marked.
The alternative is retrieval practice, meaning attempting to produce the material from memory before checking. Closing the book and writing what you remember, answering questions without looking, explaining a topic aloud as though teaching it. Each of these is harder and less pleasant than re-reading, and each produces substantially better retention.
The discomfort is the point. Effortful retrieval strengthens memory; effortless review does not.
Re-reading is not useless. It is reasonable for a first pass through unfamiliar material, where you need comprehension before you can retrieve anything. The error is continuing to use it as the primary method once the material is broadly understood.
A practical conversion is to read once, then close the material and write down everything you can, then re-read only to find what you missed.