I'm going to go out an a limb and state that neither of those actors felt those films appealed to their artistic interest or that they would contribute to their professional stature at that stage of their careers.
I dunno. I recall reading somewhere something about how DeNiro felt at one point that always playing the heavy dramatic parts, inhabiting those dark spaces, was getting to be a drag, and he wanted to get considered for lighter roles, so he ventured into areas he wasn't familiar with, perhaps not choosing wisely, but with a definite intent. He was by this time far past needing to consider the paycheque in any significant way.
As to Caine, I've heard him described as one of those guys with a need to work, not for the money, but just to be working, like a deep working class work ethic. He has one craft he has mastered, and he likes to be out using it. As to his choices, I have no idea. Perhaps he wasn't getting a lot of offers. I have read of others with that attitude - I'm an actor, I act, I don't really pay a lot of attention to what it is, I do my part. Michael Rennie was noted for having that attitude, as I recall.
