Richard Dreyfuss

The producer probably did it to him as a favor. His performance had all the subtlety of a Mack truck. The only thing missing from his portrayal of the evil Texas Republican was a scene where he ties Annette Benning to the tracks as an oncoming locomotive approaches, while he twirls his mustache.

And now a serious response;
Often with stars like Dreyfuss if they don't get over the title billing (which Dreyfess has had in movies in which he stars for most of his career) they would rather not be billed at all. This way it looks more like a cameo appearance and not that of a supporting player.

He is credited. I'm watching it right now and the opening credits clearly say "and Richard Dreyfuss."

I don't know why he was left off the beginning credits, but he is listed at the end. I agree about the reality of many actors preferring omission over something like 12th billing, but only he and Reiner know for sure.

On a related note, I liked his performance as the conniving weasel, but they placed him in the wrong party. Historically speaking, Democrats were the ones behind many of our worse times in the USA; they supported segregation, Jim Crow laws, slavery, and higher taxes. By contrast, Republicans were usually (though not always) in favor of a more limited government, and less oppression toward the people. That's my main problem with the movie: its politics and morals are completely twisted around. No surprise though, given Reiner's devotion to the Democrat side.