Why pilots use non polarized sunglasses ?

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Polarized lenses are not recommended for use in the aviation
environment.

While useful for blocking reflected light from horizontal
surfaces such as water or snow,

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polarization can reduce or eliminate the
visibility of instruments that incorporate anti-glare filters.

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Polarized lenses may also interfere with visibility through an aircraft
windscreen by enhancing striations in laminated materials (known as photoelasticity)

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     Photoelastic visualization of contact stresses on a marble in a C-clamp.

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and mask the
sparkle of light that reflects off shiny surfaces such as another
aircraft’s wing or windscreen, which can reduce the time a pilot has to
react in a “see-and-avoid” traffic situation.

– FAA

*Source:  Polarized v/s non polarized cockpit images