Why does Portugal produce so many top football managers?

When Manchester United identified Sporting Lisbon’s Ruben Amorim as the man to succeed Erik ten Hag as manager they did what dozens of leading European clubs have done in the last 20 years: look for a Portuguese coach.  From Jose Mourinho to Jorge Jesus, Andre Villas-Boas to Leonardo Jardim, Nuno Espirito Santo to Marco Silva...