I'm not refuting your claim at all but I'm curious whether you personally know of examples where an F1 visa associated with Harvard, Stanford, MIT or Yale was ever denied entry? Or even 2nd-tier universities, say Texas A&M, U Michigan, SUNY Buffalo, UIUC? Again, I ask not to refute your claim but only because I don't.
Since F1s are non immigrant visas, one could argue that if someone was entering the US to study at a community college or a bottom rung university, education may not be the sole goal of stepping foot on US soil. Hell, even the students studying at better universities are usually here for the long-term prospects, not simply the education.
I know this is a privileged take, just saying it holds water.