I don't know how much stock I'd put in self-reported reasons for attrition. At my school the undergrad professors for calc are all excellent - well reviewed by students, winning awards, etc. Still, a lot of STEM students drop out when they're taking first-year calc, because STEM programs are also just very hard. Even if we axed the first-year math requirements, I suspect those people would make it two or three years in before getting caught up on another class, and ultimately leaving anyways.