Why is Robert De Niro uncredited in "American Hustle" (2013)?

Actors frequently go uncredited in films;i.e. Dan Ackroyd in "Indiana Jones and the Temple of Doom", Kevin Spacey in "Seven", or Will Ferrell in "Wedding Crashers". They "are meant to be somewhat of a surprise cameo, not a real official role. The studio often doesnt want people to know they are in the movie until they actually see the movie, therefore their appearance comes as much more of a surprise and more enjoyable because it was so unexpected.

answers.yahoo.com/question/index?qid=20080126170329AAIlHYp website no longer exists

The Dan Ackroyd scene is on this webpage:
https://screenrant.com/indiana-jones-temple-doom-dan-aykroyd-cameo-location

De Niro's role playing gangster boss Vincent Tellegio has been widely praised by critics.

https://en.wikipedia.org/wiki/American_Hustle_(2013_film)