Why do professional green screens look so much better?

• A movie production can do another take if it doesn't look good.

• Typically, a quality TV series or movie spends $100,000s per finished minute in post production cleaning up green screen shots. The commentary in the updated Battlestar Galactica mentioned how one can tell there was money left over on an episode when there is more than a few seconds of the hangar bay background in the shot.

• The weather is usually aired live with little chance to adapt to last minute changes in wardrobe, lighting, and camera angle. Ultimately, they don't deeply care if there is fringing, bleeding, or an unintended region affected by chromakey.

• The most common mistake of green screen use is over-lighting the green screen. Its reflection on the back of the subject causes artifacts at the fringe—especially behind hair or fibers—which are hard to fix even in post production. (I know this because I once did such clean up on an art house production.) The green screen should be lit with pure green light only to improve saturation, not brightness. The effect seen in person is a very dark looking green. (I saw the set up in Untraceable, it is flawless—a scene where Diane Lane comes out of an elevator into the FBI lobby, seen from over her shoulder. The lobby was added in post.)

@wallyk's answer is excellent, and I would add just one thing: the greenscreen work you see done by local news and live TV is done in real time, unlike a movie where computers can render for hours, days, weeks etc. There's only so much polishing that even a powerful computer can do at 30 frames per second, while also pumping the rendered video stream back out to be broadcast. When the source video is recorded footage rather than a live stream, and the computer or network of computers can take more than one-thirtieth of a second to render each frame, the quality of the output is considerably higher. Entirely different software is used for not-live greenscreen work, and it produces much better results (though not as quickly).

Building off this point, starting with already-shot footage means that compositors can do preliminary work on it even before the green gets keyed out—color-correcting the image so that the green pops more, for example.

For an example of the very best live greenscreen replacement that money can buy, look at NFL games and the computer-generated line of scrimmage/first-down line that sometimes goes underneath players; or some MLB games have ads behind the batters that are added digitally, keying out a greenscreen "ad" in a stadium.

This has been said in a round about way, but here is the big difference.

Good green screening is a massively manual endeavor. To get a good key in many situations involves manual rotoscoping to separate different element to run different keying processes on to get optimal results. (For example, a different style of key might work better on a characters red hair, while another handles her blue fuzzy sweater better). This is sort of a half way compromise between completely manual rotoscoping (which Hollywood VFX resorts to quite frequently), and fully automated keying. It produces significantly better results.

In the case of live broadcast, no manual work can be done at all, which results both in sub-optimal keyer configuration and a lack of the benefits of manual rotoscoping for assistance which makes a big difference.

In the case of YouTube videos, they are probably a) using bad keying software and b) have a poorly lit green screen and c) have no idea what they are doing.

Certainly some of it has to do with the fact that Hollywood productions take the time and money to use better lighting and compositing software, but much of it also has to do with the actual screen you're using. Most small productions buy cheap chinese green screens for $50 off ebay and wonder why their composites end up looking bad. The professional film industry almost exclusively uses Composite Components Company green screens. They're more expensive but if you want a really convincing composite you have to have the right screens.