Open book need not be easier, it entirely depends on the exam.

This was too long ago for me to remember the details of but, just making an example here, we might have had a course on homogeneization of elliptic problems using a model PDE with a div(A grad u) term throughout, and then the exam was about a PDE with a curl(A curl u) and some other added terms.

Notes carry the method and theoretical tools but you then still need to apply them to a new problem.

So, if anything, our open book exams were harder and a better test of understanding and mastery. You couldn’t rote your way into a good grade.