TV's are designed to used for watching moving pictures from 6 to 10 feet (or more) away. A PC monitor is designed to be used from a couple of feet away. There is a difference.
Most large TV's have a resolution of 1380 x 768 or so, they have larger pixel sizes and don't look so good as a Computer display.
You can watch HD tv on a PC monitor, but they cost more per inch screen size than TV's.