Say what you want about preseason, but winning 4 games when you still have a playoff shot to finish the season, 4 preseason games, and your two openerss has a cumulative affect. You learn to win. The culture becomes about winning. Winning doesn't happen as a surprise, but instead, an expectation.
The Cowboys do not expect themselves to be good. They don't know how to handle winning winning the Giants game because they've done it so rarely.