Perhaps the reason why so many universities offer their students so little is they know studying at a top university remains a brilliant investment even if you don't learn anything.
或许许多大学给学生提供如此少东西的原因是他们知道在顶尖大学学习即使你什么都没学到仍然是一项极好的投资。
"why so many universities offer their students so little" 紧跟先行词 "reason",说明是哪一个原因;"why" 在从句中作原因状语,相当于 "for which",因此是限制性定语从句。