Not quite amazing, if you consider that there are literally dozens of distinct ‘accents’, even within a state, and many states have their own distinct accent. The “Southern” accent you are probably referencing is quite different and distinct in Georgia than in South Carolina and different again in Virginia.
Texas itself has a number of recognizably different accents, as do the different parts of New England, a relatively small geographical area. In the single (albeit large) city of New York there are no less than 4, and by some accounts over a dozen recognized, distinct accents.
So your point about tiny England having half a dozen or so accents is not really well-taken. Similarly sized geographic areas throughout the world probably have about the same density of differing dialects and accents, and multi-ethnic America would undoubtedly rate as one, if not the highest, incidents of different accents.
That said, accents throughout the English-speaking world are becoming less distinct as mass-media, movies and even the internet are homogenizing pronunciation and meaning.