Most (all?) animal senses have evolved firstly for finding and obtaining food.
From antelopes to bees, the spectral and polarization sensitivity (if any) of the eyes is perfectly matched to discriminate their primary food source against the background of their visual environment.
Predators evolved binocular vision (for distance estimation), and vergence tracking to estimate future prey pathing movement. In fact, amongst humans, the extremely wide margin in vergence tracking performance between the sexes is indicative of a very early specialization for hunting by men and gathering by women.
Evolving defensively against such an extremely rare event (no known extinct or extant species of snake even actively preyed against humans) would seem counter to the daily duty cycle demands of the human sensorium.