It drives me nuts too but I don't believe it due to the model of the A-League, I think its the majority of blame lies with the clubs. Constantly changing owners, football philosophies and coaches because they don't know what they want to be.
I could use any club as an example but I will use Adelaide. They spent 4 years under ex-Barcelona coaches, implementing a distinct style of play and philosophy and won all 3 domestic trophies. Then they changed tack completely and appointed Kurz who turned them in to a super disciplined counter attacking team that gave limited opportunities to youngsters. Then ownership changed (now 75% Dutch and 25% Chinese owned) and the new chairman , Piet Van de Pol, has appointed a fellow Dutchman and aims to turn the club into a developer of youth.
WSW pulled the same shit, trying to replace Popovic with Gombau (whose philosophies and football are polar opposites) and we've seen it with JVS replaced by Joyce replaced by Mombaerts. Every new coach sees the game differently and rebuilds the squad with players capable of playing their style of football.
I argue its the high turnover in A-League coaches that leads to the high turnover of A-league players.