Many vocalists won’t have a deep understanding of music theory
[1]: https://en.wikipedia.org/wiki/Category:Songs_written_by_Mich...
[2]: I'm not trying to say that being a vocalist is somehow "worse" than being a songwriter, just that removing a skill he clearly had (writing songs) is weird.
Most of the songs I went through from Jackson where he appear to be a writer, as the history of him writing and producing the song. Of course, he got plenty of help. But I don't think it's fair to reduce him as using ghost-writers/producers for his songs, people seem to be properly credited.