Why you are not able to see the flash light of AC with naked eyes?

Why you are not able to see the flash light of AC with naked eyes?
Not sure what "flash light" of AC is supposed to mean,
but if you mean the 60 cycle fluctuation of AC current ...
well, you almost can if you're watching a fluorescent light
but the persistence of vision of the human eye is such that the variations merge with each other - much like your inability to see a movie as a series of still photographs (which is what they are).

A2. Actually, the brightening cycle of a filament on a 60 Hz system will be 120 times per second, for the Voltage and the Current vary through a maximum twice per cycle. That is beyond the ability of the human eye to discern.
The comment regarding fluorescent lamps is worth noting. Our eyes are much more sensitive to fluctuations at the edges of the field of vision. That's why, when you look directly at the fluoro the flickering disappears. [Remember the rods and cones- perhaps an evolutionary advantage?].
An old fluoro tube may be pulsing more brightly for only every second of the two peak voltage times per cycle. I've never heard of this nor experienced it with an incandescent light.