Why don't sunglasses refract and focus light?!

While they're curved, the front and back are the same curve.

If you are farsighted, the lenses are thicker in the middle. (The front surface is curved more than the back.) These lenses make things look bigger.

If you are nearsighted, the lenses are thicker at the edges. (The front surface is curve less than the back). These lenses make things look smaller.

Off-the-shelf sunglasses are curved the same amount on the front and back, so they don't affect the light paths, and therefore have no correction. These lenses don't change how things look.

Try posting this in the Physics section instead.
Anyway, the sides of the glass are parallel with each other (they curve in the very same direction) like this: )). Light will pass through normally (in a straight line). If one side of the lens curved outward and the other curved in the other direction to make a () shape, that would be a convex lens which would focus light, I believe.
And I think polarization is what protects your eyes from UV rays and makes things appear darker.

Because they are ground to a zero power, just like in clear optical lenses , which can be ground from zero power to any power needed.

Optician