I may be wrong but nobody puts their best into a friendly match where they have nothing to lose if they lose. Some famous players are even careful not to get injured because they know how much it would cost them in their clubs. Some other players know that if they get injured, their home country may not make the effort to take care of them and give them the best medical attention like their clubs will, so why sweat it? The smaller countries that always win know that they are at the bottle of the ranking and would want to make a name for themselves and be recognized so the play very well. This is my own hypothesis though.