Exactly. There's also a cultural process of natural selection at work, and if humility is selected against, we shouldn't be surprised to see less of it.

We live by headlines, sound bites, quick and easy black & white characterisation. Nuance is not appreciated, and often not understood. Nuance and humility are seen as signs of weakness, and often taken as signs that you're wrong, since you clearly don't really believe what you say. A lie spoken with confidence is more believable than a truth explained with nuance, and that's a real problem.

You're right that there are few rewards to humility.

An example that immediately comes to mind is during my thesis, I set upon the task of creating a multivariate model to predict an observable quantity. My advisor kept insisting that my goal was NOT to build the model, but to first see that IF the variables had enough predictive power. I just never understood (at the time) what he meant. I went straight ahead and hammered out a robust model, and got a good grade (reward).

The rewards are intrinsic, not extrinsic. Sometimes the rewards can be extrinsic in the long run - if you find and invalidate a hidden assumption and are then able to build a theory/product/business around it. But in the short term, the rewards, and hence motivation, is usually intrinsic at best. Often there are no short term rewards, because you're in a perennial state of discomfiture owning to questioning everything.