Lenovo has a plastic bezel around the screen which makes it easy for them to add this feature [1]. I am not an Industrial Engineer but it would seem much more difficult to do this with glass and not impact the structural integrity of the screen.

Having just had my screen be ruined by a micro-fracture I would rather just trust the green indicator dot than be without my laptop for a week.

[1] https://www.businessinsider.com/lenovo-thinkshutter-laptops-...