Maybe the question should be: Why doesn't the high end audio power cord makers, produce a power cord compatible with that small socket found on most DVD players ???
Dan Wright at Modwright.com adds the large IEC as part of his DVD player mods. I think it is really an economic $$$ issue. Joe and Jane Consumer could care less; plus, the larger 6 foot 18-3 detachable power cord cost more than the lamp cord they give you. The larger IEC costs more also; and they would need to train the workers and stock more parts. Multiply all those costs by the millions and millions of units sold. The competition is tough in that market and razor thin profit margins matter.
I know the Cary and Camelot Roundtable DVD players have the large IEC power cord socket.