Your “easy” circumvention requires a second monitor, a teleprompter mirror, a camera, a separate computer and a video capture device. Remember, most people who use Teams don't work in tech. And for them, that’s not “easy”. Barely any normal people have that kind of equipment just lying around, along with the expertise and desire to set all that up, ready for a teams meeting.

For every 100 people who might decide to take a screenshot during a teams meeting, I doubt there’s 1 person who has all that equipment set up and ready to go. You don’t need to make something 100% effective to get a benefit from doing so.