Grade inflation is real!
According to a recent article in The Atlantic, "The Two Most Important College Admissions Criteria Now Mean Less," more than half of US high school students graduate with an A average. Over half!! And affluent suburban high schools are the biggest grade-inflation offenders.
Imagine that you are a college admission representative at a highly competitive school. You are looking at transcript after transcript of straight As, and often exceptional test scores as well. There is little to distinguish one applicant from another.
So while great rigor, grades and test scores are necessary to get you to "the dance," it is often the other parts of the application that help to make a decision.