The UK principle has always been that the examiner sets the standard. If the examiner fails to do that it is the examiners credibility that is on the line.
The only way that you will have a completely impartial examiner is if his income does not have any correlation with the outcome or number of tests conducted. That is why the CAA retain the examination of initial IR candidates.
Many countries do what you propose with the result that getting a test can be a nightmare with waits of several months. Are you aware of how many examiners there are, and how many people would be needed to centrally program them? Then take into account cancellations, re-sits partial tests etc. The cost would escalate with little or no overall improvement in safety.
Surely it would be more productive to standardise, and reduce the overall numbers of existing examiners. Many regard it as another tick on the CV and have no real interest in testing.