First appearance: More Fun Comics #51 (1940)
Despite being an all-powerful, omnipresent spirit of vengeance, the Spectre just doesn't have the type of personality that lends itself well to solo adventures. His stories are always overbearingly serious, and he relies a lot on his monotone speech and cryptic dialogue to get his point across. Over the span of a two-hour movie, that might wear a bit thin.
There's always a risk when adapting supernatural characters for the screen because so many of the plots and personalities are completely unrelatable for the audience. Unlike Spider-Man, Batman, and Iron Man, Spectre doesn’t have a whole lot for most people to identify with, let alone a dynamic personality. If he ever somehow made his way to film, it would probably be in the form of a Ghost Rider-esque affair. Take that as a fair warning.