We can't just make the tests hard enough so preparations will have diminishing returns? When I look at the exams in elite universities like Tokyo University, JE

We can't just make the tests hard enough so preparations will have diminishing returns? When I look at the exams in elite universities like Tokyo University, JEEs, and etc, I'm not sure how preparation can help the students in different talent pools. An extreme example would be those selective STEM summer camps. For instance, students in SUMaC will pick a subject in number theory, abstract algebra, or algebraic topology to study for three weeks. I don't think everyone can even understand the topics no matter how hard they try.