There is a Service Time component of this too. Generally the league is getting younger but you also don't want them to be a free agent at age 26 because then it gets more super expensive.
There used to be a learning on the job component but now teams wait until they are 110% sure he is ready so they get all of his prime.
The worst case that happens is a top prospect sucks for 3 years like byron buxton and then becomes good because then you just paid for his education for someone else. Because of this teams like tampa with not a lot of money wait long for the call up of top prospects.
Nonetheless there were many young great players in the last years but you need to consider that for a hs guy to make the majors at age 20 he needs to make it through the minors in just two years which is super short for a hs player.
Most hs prospects take 3-4 years and most college guys 2-3 in the minors. Because of that most sub 20 guys are international.