Why do snakes have no movable eyelids?

Snakes do not have movable eyelids because their eyes are protected by a fixed, transparent scale known as a spectacle or brille.123

This transparent cap is a specialized layer of the epidermis that covers the eye, serving to protect it while allowing the snake to see. Because this structure is part of the outer skin, it is shed and replaced during each molt. While the exact evolutionary function of the spectacle remains a subject of study, it is believed to help reduce water loss through evaporation from the head region. This feature is also found in some lizards, such as geckos, representing an example of convergent evolution.1234