Why are actors sometimes uncredited for example Tom Sizemore and Seth Green in Enemy Of The State?

In old films, some minor characters might not have been listed in the credits. For example, someone who moves a patient from one room to another. Today there are multiple reasons: The actor hates the film and does not want to be associated with it. This is more common with directors than actors. A big star might like a particular role but it's too small for his star power. An actor is just doing a favor for another director or actor who is involved in a production. It could be an inside joke for the fans.