>well-developed top-flight leagues

It's not about this at all. The best developed leagues are England, Italy, Spain, Germany, France in that order. On top of that, you have different levels of inequality within leagues, for example huge in France(dominated by PSG),big in Germany(Bayern slightly less dominant) and Spain(a few big clubs) and small in Italy(lots of pretty strong teams) and England(strongest "weak" teams). A Spanish player will choose to play frequently at a top Italian club over sit on the bench at Real Madrid.

The other really big factor, the reason why Saudi and Qatar and to a lesser extent England have so many domestically based players is money relative to talent. Also why Brazil and Argentina who have popular domestic leagues have so many players playing abroad. Almost every league has some sort of cap for foreign players or quota for domestic ones. The exceptions are Germany and Portugal without quotas or caps and Brazil where there is so much talent there's very little need to ever import players. Asian leagues generally have very restrictive, 3-5 caps on foreign players. The gulf leagues especially have a lot of money. Thus, a decent domestic player is worth their weight in gold. In England, you need to have a certain number of domestically trained players, a hangover from the EU days as you cannot directly discriminate against players from other parts of the EU. There is far more money in English football than there is in European football, and as such a good English trained player, most of whom are English or British, is worth a lot to English clubs.