I wonder if it hasn't gotten this way simply as a synthesis of multiple agendas and their outcomes.
Management wants to hire the cheapest possible talent. For a while, they achieve this goal, and the staff mix shifts towards less talented people. Productivity expectations don't go away and the more experienced/better staff shoulder the burden.
Management notices (perhaps even having to fire some quantity of cheap staff for obvious gaffes and lack of productivity), and listens to the chorus of "hire smarter people". So the interviews get harder, with the idea that this will allow smarter *and* cheaper hires.
This works to a degree, but now the more experienced people are somewhat threatened by an influx of smart *and* cheap staff. So the interview questions get much harder and more unrealistic under the guise of ever-smarter people requirements, but the actual goal is just raising the bar so that you wind up with really smart people but too far out on the spectrum, deficient in the soft skills that would allow them to rise in the organization. The old hands gain talent that eases the workload but greatly reduce the risk to their own organization standing.
Even if none of this is true, it still seems that an obviously flawed hiring process like this has to be a byproduct of agendas other than simply "hiring the most capable people". Still I think cost and self-preservation are probably large factors in these processes.