Why are so many silent movies lost?

There are quite a few reasons.

1. Vandalism. Studios viewed older films as worthless and junked them. For example, Universal purged the last of its silents in the late 1940s.

2. Neglect. Improperly stored films rot. For example, an archive let Colleen Moore's collection decompose because they forgot to store the reels.

3. Fire. Nitrate is flammable. Studio vaults and archives still regularly go up in flames. Theda Bara's filmography is gone due to fires at Fox and MoMA.

4. Popularity. Every time a film is run through a projector, it is damaged. It is possible to literally play a film to death. For example, Oscar Micheaux made high demand pictures but could only afford to make three or four prints.

5. Apathy. Many film companies have gone under. Unless measures were taken by somebody (anybody) their catalog would rot, burn, vanish.

6. Embarrassment. Mamy silent stars were gaslit into believing that their old work was ridiculous. Mary Pickford had to be talked out of destroying her own films.

7. Politics. Archives were sometimes invaded and films deemed seditious, socialist, decadent, lascivious or otherwise Bad™ would be destroyed.

There are other reasons for list films but there are also miraculous tales of recovery.

The Dawson City find of reels under an old swimming pool. The improbable preservation of LIMITE and FROM THE MANGER TO THE CROSS thanks to one determined person etc.