Fame
Producers are strictly behind the scenes. The power behind the throne (and by "throne" we don't mean toilet). The guy behind the guy behind the guy behind…you get the idea.
Hardly anyone outside of the film industry knows the names of even the most successful producers. Only a handful of names throughout the entire hundred-plus year history of filmmaking have ever become commonly known: Walt Disney, Aaron Spelling, Jeff Katzenberg, Jerry Bruckheimer, George Lucas, and very few others.
Fame isn't something that the job usually affords. But when you're doing your best Michael Phelps through an Olympic-sized pool of cash, who cares if the average person knows your name?