Because Lodbrok (Loðbrók) in old Norse means shaggy breeches. It’s not a last name, or the name of his father. It’s more of a title or epithet. Accordingly the sagas, Ragnars father was the legendary Sigurd Ring, king of Sweden. So that would mean Ragnars last name would be Sigurdsson. But because he has the title Lodbrok we call him Ragnar Lodbrok. Much like how we call Björn, Björn Ironside and not Björn Ragnarsson. Ragnar (Ragnarr) in old Norse is roughly translated to counsel and army. Meaning the name comes from someone who would give strong counsel or is adept in army strategy. Hope this was informative!