How though? He's won and won impressively wherever he's been, so I'd be interested to know what more he could have reasonably done to obtain these top European jobs you allude to. You touch on an important point here, because attitudes like these highlight the barriers which prevented a clearly very good coach from getting an opportunity in Europe for most of his career.
Take his time at Brisbane Roar for example. He takes over one of the worst teams, completely overhauls the squad and wins the Championship in both of two full seasons that he's there. He sets an Australian record in not only football but any sport for the longest unbeaten run, which still stands to this day. He introduces a style of football which essentially changes the way football is played or attempted to be played in the country.
And yet despite successes like this wherever he's been, how is that meant to be translated into an opportunity in Europe when such achievements are summarily dismissed because 'it's only Australia' or 'it's only Japan'? He was highly recommended for the Sunderland job at one stage and their chairman went with Chris Coleman, later giving the reason that he needed "a big name for the fans", i.e. somebody they'd actually heard of.
It's kind of a Catch-22 where he can't get a European opportunity because he doesn't have European experience, but he can't get European experience because he can't get that opportunity. Which is why this argument against him of "well if he was any good he would have been managing top clubs in Europe" is naive, and frankly a bit dumb.
What is clear is that there are very, very few pathways for an Australian coach or coaches from similar regions to get an opportunity in Europoe - Postecoglou is the one creating those pathways.