There is also a financial incentive here. Hollywood actors who are in the twilight of their careers or have seen failure at the box office make a desperate effort to be relevant and competitive and they know who butters their bread in hollyweird. Adhering to liberal dogma will get you invited to the right parties, connect with the right producers, participate in lucrative activities that will bring them much needed exposure and financial rewards. I don't believe that actors are ideological in the pure sense of the term. There are those that are, Matt Damon, Alec Baldwin, George Clooney come to mind. But the vast majority simply do not have the intellectual heft or the necessary intellectual framework to be ideologically driven, they simply follow the hollywood zeitgeist in the hopes that they are rewarded hence they adopt liberal viewpoints. They may simply be acting the part as liberals to achieve their goals.
Deniro sadly has left the station a long time ago.
Good summary of the movie "What Just Happened" in which DeNiro starred.