I think people often ignore the fact (well, not a fact, my opinion) that for many of these players, blossoming is playing first team football. Had United signed Mangala a couple of years ago who knows how many games he would have under his legs? Certainly very few, with so many more experienced players ahead of him. He is now an experienced CL footballer. Would United also buy a player (with unproved value at the time) like Falcao or Jackson Martínez and give him enough minutes or he would be a second/third choice for years on end? Going straight to the top clubs is not necessarily the best option for most players.
Not to forget that the working visa thing makes these things more complicated for English clubs as well.
A few noticeable exceptions exist. A player like Moutinho was sold by Sporting to Porto because they never had a better offer for him and he was pushing to leave. A classy player all his life since his debut at 17 or 18, spent 5 years at Sporting and no one ever noticed him or made a decent 15m offer for him (we got him for 11m). Criminally underrated throughout his career. Porto sold the likes of Meireles for 15m to Liverpool and replaced him with a much better 4m cheaper player. I would guess football clubs had professional scouts watching these players week in week out.