Why is peer review so random? 8 Scientific Papers That Were Rejected Before Going on to Win a Nobel Prize

Funding Analysis: Researchers Say NIH Grant Funding Allocation Seems No Better Than Lottery

The same paper resubmitted to the same journal after several years often ends up rejected due to 'serious methodological errors'

For people whose profession revolves around making order out of seemingly-random observations, scientists sure are inconsistent at judging the work of other scientists. Why? It certainly doesn't seem to be like this at all levels. For example according to the GRE's website,


  For the Analytical Writing section, each essay receives a score from two trained raters, using a six-point holistic scale. In holistic scoring, raters are trained to assign scores on the basis of the overall quality of an essay in response to the assigned task. If the two assigned scores differ by more than one point on the scale, the discrepancy is adjudicated by a third GRE reader. Otherwise, the two scores on each essay are averaged.


This implies that it's uncommon for two assigned scores to differ by more than one point on the scale, i.e. GRE essay raters usually agree. Similarly, as far as I know, undergraduate thesis readers, MS thesis readers and even PhD thesis readers don't usually come to diametrically opposed judgments on the piece of work. Yet once it gets to research-level material, peer reviewers no longer seem to agree. Why?
Good question. Hard to answer. Some thoughts:


reviewers are not trained
reviewers are anonymous
reviewers receive minor feedback on their performance
reviewers are also authors, competing for the same funds/ prestige
reviewers are specialized in a narrow discipline
reviewers are volunteers
reviewers are scarce
the review system lacks an external (independent) control system (audit) 
reviewers are humans, with their own personal interests, emotions, capabilities


Considering these observations, it is unlikely to expect two review reports to be aligned. Then the difficult decision transfers to the associate editor who is also a volunteer and not specialized in the author’s field.

Leaves the question why it is accepted while outside science this wouldn’t be. Honestly, I don’t know. Just some guesses:


Science is a powerful isolated sector with its own rules?
The current system works for established research groups?
Jourals do not have the funding to train and attract qualified professionals/scientists as reviewers?
There is no easy solution or alternative?


Added based on comment:
- reviewers are busy scientists
- reviewers are career-wise not rewarded for conducting reviews