In Poland it is because scouting in lower leagues is shit. There are a lot of talent in the Polish lower divisions but the scouting system of the top clubs are really poor. Lewandowski had to score a lot of goals in the lower leagues before the top clubs got interested in him. Legia eventually signed him but he wasn't being given any playing time and so Lech who do actually have good scouts, decided it was worth bringing him in. There is a good article on his story I could dig up if you guys want.
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Perisic 3-1.