(1) They can try. The results vary. Certainly, hiding the "countless" examples behind the *best* examples is overestimating the quality of all examples.
(2) Some literature *relies* on the texture of personal experience. Some doesn't. There are great examples of both.
(3) A white writer can transliterate dialect flawlessly, research relentlessly, document extensively, and sympathize a whole bunch...but second-hand is second-hand. It works in all directions: black writers' attempts to get into white people's minds can be pretty jarring. To pick an example, Ernest Gaines wrote in the voice of a white college student in a chapter of A Gathering of Old Men...and I didn't think it worked as well as the other voices.