Why can't you take a picture of television?

You can; just don't expect great results. If it's a tube type TV (CRT) the image is refreshed as a series of interlaced horizontal lines at a rate of 60 times (I think) per second. If you use a shutter speed of 1/60 or less, you will capture one of those "bands" of lines for sure. If you use a slower speed, say 1/30 or less, you should be able to eliminate the band, but any motion on the screen will be blurry. As for digital, pixelized TV, I have no idea what you'd be up against.