Lots of HD broadcasts still have noise around the edges (it’s not noise, actually, it’s an alternate data channel stupidly embedded in the image), but it’s a tiny fraction of the screen. Broadcasts have certain defined values for title-safe and action-safe areas (how close you can put stuff to the edges of the screen). For HD it’s SMPTE ST 2046-1, and for SD it’s SMPTE RP 8. It’s not a matter of some crazy content creator deciding to put in unnecessary space around the edges, it’s a defined industry standard that some people simply follow more religiously than others. Not only that, but some broadcasters require that their HD content have extra-large SD-sized space on the edges. Heck, most content creation tools still use the SD-sized values. Why? Because some viewers are watching HD content on a 30-year-old CRT SDTV, and they get calls at the station when critical text is cut off. Add extra space and the phones stop ringing. There are still tons of old SD CRT’s actively in use in the world, and they are built to outlast us all. Skew the demographics to the 75+ crowd and you’ll see where there’s a perfect storm of old CRT’s, cable TV, low tech literacy, and irate phone calls.
Not all content follows these rules. Games for example like to make use of every available pixel. And that’s why most TV’s allow you to change overscan on a per-input basis. Turn it off for Blu-rays, games, and PC inputs, leave it on for the cable box. I hate that it’s on by default too, but I see why they do it. I’m just glad it’s so easy to turn off.