How are you supposed to assess whether a student has learned quantum mechanics, or chemistry, or calculus if any questions you ask them they can simply contact another student for help, or get someone to do it for them on sites like Chegg? Short of giving an individualized oral exam, which is not feasible for anything but tiny classes, proctored exams are necessary for assessment in such classes. This has nothing to do with memorization or time pressures, and everything to do with making sure the student is the one who actually did the work. In an ideal world one could trust students to do take-home exams on their own, but I’ve yet to be at a university where undergraduates did not take advantage of such trust…