[YoshiRulz]

This was addressed at the end of the video: The main points are that it can't be used outside, or when the shot includes the subjects' legs/feet (as the ground would need to be an amber-screen as well), and on top of that, it requires a certain level of setup which some shoots can't afford.

[laserbeam]

I don't think I've ever seen green screen used outside either. I think you can kinda use it outside... But any random cloud passing by would destroy your morale in the editing room if you're keying against green screen. Also lighting conditions change during a day and you'd have to be aware of that when keying. Not fun.

[niccl]

Lord of The Rings used some huge external green/blue screens. One I know of was the scene in Return of the King where there's a dead oliphaunt and, IIRC, Eowyn fighting the Witch King. But there were lots of others.