I'd wager the main difference between "many decades ago" and mid 2000s onwards is the perceived stakes of college. My time in college (around that time) was perceived by most as "make or break": either you did well in college, or you were doomed to a sub-standard lifestyle (not to mention the debt of college tuition).

Obviously, whether this was true or not is a whole discussion, but the attitude did lead to a lot more cheating (due to desperation) than I'd imagine past generations had.

A midterm being worth 25-33% of a grade, plus some classes only being offered in fall or spring semesters meant a bad test could roughly cost you tens of thousands of dollars, since the next time you could retake the class would be in a year, and it often was a prerequisite for another class. It just leads to an environment that encourages desperate "survival" behavior.