As someone who has attended this kind of program, it's because some students will cheat and view proctoring as an annoyance.

Imo it's both on the students (plenty of students are optimizing just to get a class out of the way to do more interesting stuff) and the programs (some classes just aren't up-to-date or are rightfully viewed as busywork).

Personally, I found courses that were output heavy and regurgitation light tended to be the most successful from an honor code perspective - you can't cheat your way out of "learning by doing" when you are held accountable for the output (eg. A research grade paper or implementing a fully functional Linux kernel).

Sadly, even at Ivies most lower div classes are just rote memorization because class sizes would be massive for plenty of core classes (100-500 students for some classes).