Why are so many older horror movies simply lost

One thing I started really getting into is learning a lot more about older horror, movies especially with people like James Rolf, who is a big-time classic horror movie fan. But like a lot of times, it makes me think why are so many of these movies lost to time like the original Lon Chaney Phantom of the opera that people saw in the 1920s in the theater. or like London, after midnight, probably the most famous lost horror. Movie of all time just simply doesn’t exist. I was told that a lot of times that movie theaters didn’t really preserve these movies and when they were done running, they just simply threw them out and like a lot of these big production companies like MGM or Paramount store them in vaults and because the film was so highly flammable, that’s why you had a lot of film vault fire. But I will say the cool thing is that every once in a while a lot of these movies are lost and then all of a sudden some private collector has the entire movie or even a collection of them