I assume that if Finale were open-sourced, the code would be so ugly that nobody who worked on it would ever be employable again. Finale also has some tight integration with its Midi player and audio output that may have some weird patented shit or Apple/MS proprietary jank.
Indeed, I have heard that at least early versions of Finale had painfully unmaintainable code that severely slowed its development.
This supposedly was/is in part because Finale's original author Phil Farrand [1] was a musician turned self-taught programmer and Finale was only his second software product.
