Well, for both grades and GRE scores, the assumption is that you're going to do at least decently: if you had truly terrible grades, then that would raise a lot of questions about your work ethic / interest in the subject / etc (those questions might be addressed if you had strong letters of recommendation from well-known professors who explained why you did badly, for example).

But certainly the top schools don't simply take the students with the best GRE scores or marks: there are many rejected applicants with perfect GPAs and great GRE scores, but no research experience.