When taking a test they have a proctor that's watching you on a webcam, and they make you pan the webcam around the room to ensure that there's no obvious way to cheat, and they make you share your screen to ensure you only have a browser running.
It’s perfect for all parties, and doesn’t intrude on your personal living space or devices.
Well that level of intrusiveness would just make me come up with something overly complicated just so I could prove that I could cheat if I wanted.
That's why they make you share your screen; obviously there's plenty of ways to fool that but the goal is to make it so that cheating requires enough effort to where it's probably less effort just to study and do it right.
They just want to make it difficult enough to catch most.