Movie producers produce movies with an explicit agenda in mind. They do not produce movies to entertain.
The movie has a clear, definite idea that it wants to implant in the heads of the viewers whose critical reasoning facilities are softened by public school and the expectation of being entertained.
It is not the other way around: an entertaining movie has a few agenda-furthering blurbs here and there.
One counterexample, in my view, is Woody Allen.
In fact, whenever anything explicitly political surfaces, it seems vaguely out of place. I get the feeling that its insertion was more obligatory than agenda-driven or artistic.