When uniform is not prescribed for sim sessions people come in a wide variety of clothing which can make the atmosphere very casual. In a Simulator session a lot of play acting is involved to make it life like. Wearing uniform can be the first step because you don't fly without uniform. Then again it's up to the airline. Pilots have to be loyal to the pay check.
Perhaps authoritarian, steep-cockpit-gradient companies that have had CRM issues might consider
loosening up their dress standards...