People need to stop saying this. There are a few brands of Laptop (Dells Alienware 17 and 18 series for example, Sagat as another example) that have both HDMI output and Input.
That link, you can read the technical specs on the 17 series laptops and they confirm Input. I also own a 17 series laptop and use the input to play on my PS4 when my girlfriend uses my large monitor to watch television.
MOST laptops yes are output only. but there are a few brands with input. DO your research people.