There's a very simple reason and I predicted this back in the 1970's.

Science is wrong. They are wrong about a great many things because the definitions and axioms are wrong, Of course this isn't the sole reason that progress has slowed dramatically in cosmology. Here the problem is shown by the failure to find the unified field theory and it largely caused by the inability of the tool being used to progress. Experimental science, reductionistic science can not by itself get over this hurdle. I believe the only alternative will prove to be the operation of reductionistic science in tandem with observational science performed by machines. We will have to back up 4000 years before moving forward.

"One explanation that I have head is simply science is
reaching the point of demising returns. The easy stuff has
already been done. It is taking more and more effort and
more and more sophisticated technology make the same
advances."

The nature of the tool we use is highly dependent on its axioms and definitions when it has progressed so far as it has. This means we must use new techniques rather than giving up. There will be much more new technology going forward but this will slow and stop as well as technology catches up with theory that is standing still. This is the largest threat o our species as those with evil motives use the new technology for their own interests.

If we stop we die. We must move forward even if that means revisiting the past. These are perilous times when only the few can afford to buy science and they demand a single norm and a single answer. Then that "science" is used to excuse public policy that helps only those who bought the science.

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Man fears the pyramid, time fears man.