Which is precisely why when Tolkien's publisher wouldn't print it and made him write the Hobbit and LotR instead. The Silmarillion was finally published by Christopher Tolkien after his father died.
Like you, I really can't imagine it making a successful movie franchise. The scale is too big, the stories too depressing and unsatisfying, the relationships too complex. It makes a better fireside tale than H'wood hit.
One thing that is little known about Tolkien is that he was part of a writer’s club called the Inklings. C.S. Lewis was also part of the club.
One of the things they would do is pick things to write about. In this case, Lewis choose to write a story in regards to time. I believe that C.S. Lewis had to write his story based on a place, his wardrobe.
That’s where Narnia and LOTR had their origins.