The issue is, and its one that our owners are quickly having the learn to deal with, is that real football isn't like Fifa, where a player has a rating and they are that good. You simply buy the player and they are that good for you too.
I get your point on having good environments for players to develop but I think its more than that. Some players settle in a country, some players are better at fitting in where they dont have to be the star, some players thrive being the star, some players hate a certain manager, some players love a certain manager. Theres a whole world of human psychology that goes into players being great. Yes the ability has to be there to an extent, but how its applied and used is a whole other world.
My favourite example is Rudiger and the way he was just completely transformed by Tuchel. Tuchel's personality and approach to the game screamed that Rudiger would thrive. Tuchel took the animal that was in Rudiger and encouraged it, where others had tried to calm him down. I can imagine he would have done the same with someone like David Luiz. You can also say the same for some of Liverpool's squad in the period where they were keeping pace with City. Players like Robertson and Mane, Henderson and Van Dijk all bought in to Klopps game.
But for all of these who buy in, there are players that don't. Minamino is one for Liverpool, you could list numerous for Chelsea under Tuchel, Conte Mourinho, even in the squads that were successful. Even city have had players like Ferran Torres and Kalvin Phillips that didn't stick.
Its not as simply as to say, these players from Serie A would thrive under Pep or De Zerbi. They might, sure. But I would wager that a lot of their recruitment success is born from understanding the type of personality they need to feed into their culture.