Because they were shot on film. Because a film image is not made up of pixels, there is no limit to the resolution you can convert it to digitally. Go look at films by Georges Méliès, one of the most influential filmmakers in the days of early cinema. His films can be found on YouTube in HD, sometimes in 4k, despite being shot almost 100 years ago, because his work was shot on film. When converting film to a digital format, the resolution of the film itself is not what determines how large of an image can be produced from the conversion. It has to do with the quality of digital camera or scanner being used to scan the film cells. If a 4k camera can scan a film cell, then an image can be converted from film to 4k digital video.
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