One of the reasons I have heard for the actual master tapes no longer existing is that they wanted to save money, so they erased and recorded over the original music. I find that pathetic. Others were lost to fires, plumbing problems, being tossed in the trash and bad storage.
To be sure, a lot of recordings came out of the '70s and '80s that really were bad. They could have been so much better but part of the cause was drug use. I know of one band that had a short-lived contract with Electra Records and one of the band members told me the producer had parties and bought coke, paid for with the money from the band's budget. They were really good, too. Somehow, a third party got the master tapes and remastered them to re-release them on CD. The original recording on LP , produced in SE Wisconsin at a decent studio, was very strange-sounding WRT the drums. It was anything but 'open-sounding', but the other instruments were OK. The remaster is much better and the drums don't sound the same, only similar. The original drum sounds weren't very good, unfortunately. If I were to describe the sound for the snare drum beats, it would be 'splut'.