Two things essentially:
1. Monitors don’t have TV stuff, such as tuners, sometimes speakers, image enhancements, and tons of inputs for the many things you often hook up to a TV. Basically monitors aren’t meant to be the screen sitting in your living room and/or hooked up to media. Most cheap monitors don’t even have to look good – they’re typically washed out and have wonky lines. But what they’re good at is making computer stuff look good. A lot of decently-priced monitors have high refresh rates and make sure everything is pixel-perfect as the computer intends to display them.
2. TVs only need to be good at displaying things for you to watch at your leisure. That means typical 60Hz refresh rates, not so accurate color but vibrant colors, and image processing stuff like soap opera mode and sharpness. Even game modes aren’t standard, and even then the quality of them can vary. Often times, to make a TV behave like a monitor, you’d have to turn on game mode and turn every image enhancement setting to off.
Therefore, in normal home/office use, you can use a TV as a monitor, and a monitor as a screen for whatever media you can choose to hook it up to. It is when you do more serious computer stuff and more serious TV stuff that the differences start to really matter.
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