If my sunglasses provide "100% UVA/UVB Protection" why can't look directly at the sun?
The damage to your eye does not only come from UV exposure, but also from infrared (IR) radiation. Your eye contains a lens that focuses the incoming rays to a narrow point. This point would get very hot if you looked directly into the sun. This is comparable to focussing the sun on a piece of paper with a magnifying glass, it would start to burn. Also, the rods and cones in your retina are only tolerant to a certain exposure of visible light. 

Source: https://eclipse.gsfc.nasa.gov/SEhelp/safety2.html