Why exactly does selecting and testing work better than grooming (and breeding)
Assuming it does,
Several factors may come into play and selecting may not be the only thing that is different between our current society and say a medieval society. Quantitatively, how much of a part does this one play in our current economic success?
That being said, we also have a pretty large pool of people to select from nowadays (stemming from for instance, our total population being larger, leading to more outliers in capability/skills, and from better communication, transportation, etc. which allows to search for appropriate candidates wide and far.).
Also, maybe our ability to select has grown faster and better than our ability to groom/breed (and this was at least in part coincidental and not actively pursued—see above about population size), while our capacity to groom/breed may have stagnated. The quality and efficiency of education may be better than what it used to be back then for groomed elites—though I don’t know if it is for sure (what science knows and what can be taught would appear to be better now than then though) . Our ability to “breed” doesn’t seem to have improved much (eugenism is a dead idea). I’d actually expect that eugenism and genetic engineering could fill an arbitrarily large part of that gap if it was actively pursued (which it may yet be, the debate about CRISPR-Cas is still hot, and places like China may well push forward with such ideas).
Like many people I felt compelled to distinguish myself by solving your problem while playing by your rules (rules which aren’t completely clear). But after all … and I guess I should offer an apology if this doesn’t help, but, why should any of that change anything? Picture someone who for his whole life thought he had free will, then discovered that the universe is deterministic, with all that entails about ideas like “free will” as normal people envision it. This sounds pretty similar to your situation. You discovered that you may at any point “become” or “jump” to another conscious being whose memories are consistent with your own, but whose life and environment/universe is vastly different from your current/own universe.
Then what? What are your goals anyway? How does that change of perspective affect them? How can you best act, adjust yourself to still pursue those goals? What else should matter to you? The waters may be a little muddier than you believed them to be before, but not so muddied that it should be impossible to move forward. Seriously, aside from the vague existential angst, elaborate how this change of perspective affects your beliefs, and what actions you think you should act to reach your goals in life (if you have a good grasp of your goals. If you don’t, then you should solve that first).