This is largely a cultural thing.

Japanese players would mostly be embarrassed by being the one to bring a group of random people down, and so they practice in groups of people they know and then take what they've learned to the Duty Finder and "pickup" groups. English speaking players, on the other hand, tend to believe you should play more seriously with people you know, and that the only way to really get things done is in statics, Free Company groups, etc. Both methods work fine, but when there is a heavy bias toward one way it sort of forces it to be the only way to go (i.e. a large number of people believe only statics can clear things, so all the best people join statics, thus making it so only statics can clear things).

This was also true in FFXI. NA players firmly believed endgame content like Dynamis and Salvage was designed for tight-knit groups to work on, but if you looked at shouting and search comments during JP prime times they were doing those all in pickup parties.