Why do you think so few artists attempt to create a magnum opus? It really seems most people who are good at something want to just keep making it in various forms, but don't try to make a single lasting great work, even when they're in a position to try.
As an example of what I mean, take Virgil who became well known for pastoral writing, but always had an eye on writing a national epic. A few people still read Georgics and Eclogues, but the Aeneid is the Opus.
