Preserving the source for every movie would be absurdly expensive. That can easily be many terabytes each. Textures and models are huge if any CGI was used. Not to mention all the raw footage... Wouldn't be surprised if most studios just throw that stuff away a few years after release. Why would you need to re-render the movie?
They still had source negative film for Star Trek: the Next Generation in 2012, 18 years after it went off the air. They used it to do an HD remaster for a show originally mastered on video tape [0]. I don't know if this is common or rare.
