Because "know more" isn't actionable. Knowing more is achieved by studying but not necessarily more time spent studying, but well spent effort. Staring at the page for hours and saying "I don't understand" doesn't help. Solve exercise problems, explain the material to fellow students, discuss it with them, make mind maps, bullet point summaries, work through derivations step by step, etc. There are many techniques.

At the end of the day though what matters is what you know. Furthermore, if it's a serious subject, it shouldn't matter whether you learned it from this teacher or from another school and teacher, as long as your knowledge is correct. Knowing the idiosyncracies of this particular teacher should not factor into the grade. A serious subject can be learned on one continent and examined on another. Bullshit courses are all about learning pet peeves and hobby horses of a particular teacher.

I would usually say something along the lines of “everything we covered is on the table” or “everything we covered since the last exam is on the table” depending on the nature of the test. That’s the same message as “know more” but I think it sounds politer.

You become a person who knows more, by studying more about the things you need to know.