That is often given as an excuse for this common experience, but it is not really intended. Top schools are assessed on their graduation rates. It is part of the rankings they are obsessed with. It serves no one--students or schools--for students to be weeded out in these intro courses.

The truth is just that even top schools in the US (and I suspect, ESPECIALLY top schools, more so than middle-tier schools) are really, really bad at teaching STEM, even to students very interested in learning. I still have clear memories 20 years later of my awful Calc 3 (multivariate) class. In one lecture, the prof spent the entire time on one problem on the chalkboard. He never looked at his notes. At the very end, he looked at his notes and then at the solution on the board and then back at his notes...and stuttered that the solution on the board was incorrect. Somewhere along the way he made a mistake. "But you get the idea," he said. No, we didn't. Just awful. We were completely on our own. All this guy cared about was his research. We were a distraction to him.