Question

why do sunglasses have no power even though they have curved surfaces?

Answer 1

they are used with the principle of reflection of sun rays by convex mirror.

Answer 2

I'm quite what you've noticed is the curvature of your shades.
But what you haven't noticed is that both the inner surface and outer surface of your sunglasses is curved by the same amount.
For power to be realized, light needs to bend significantly. Which doesn't happen in case of sunglasses. Happens, it does happen but not to an appreciable degree for us to experience the power of the glasses
You might have studied lenses i guess, notice the difference in curvature of both the surfaces....or you might say that the thickness of the lenses vary.

Excellent question btw :)