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).