Because a TV is not the same as a dedicated computer monitor and you are limited to 1920 x 1080 pixel on a 30" or even bigger TV, which looks abysmal.
There are cheaper alternatives to the ACD, look at Dells or other manufacturers.
Not to put too fine a point on it, but the 27" Cinema Display has enough pixels to display four (4) simultaneous 720p HD video streams.
I might add that computer monitors have fundamentally different design goals than TV sets. TV sets are designed to be viewed from across the room. Computer monitors are designed to be viewed from a chair that is within arms reach of the the monitor.