Because computer monitors don't have video processors in them, this makes them very poor at sorting out the picture and making it look good.
If you watch video on a computer (using a computer monitor) it doesn't look bad because the computer is sorting out the picture and then sending it to the monitor. But you are talking about feeding it with external source which is also very unlikely to have a video processor as they expect the monitor to do it.
You can get some computer monitors that have TV tuners in them, so if you are desperate for that idea then I would go with that, they often have a lot more inputs so you can plug in a cable/digital tuner.
Also if the TV has component in then it might have a video processor so wouldn't be so bad.