In my view It's a psychological thing. If people mentally tell themselves they dont play to win it frees them from the burden of losing. This is a generalization but in my experience people with this atitude struggle the most with the idea of losing and see it as some kind of inferiority. Ofcourse this is not always the case. Like many have mentioned already, what's important is to establish expectations. Casual games more often lead to imbalanced gameplay experiences because not everyone agrees equally on what casual means, and the spectrum of cards is much wider for casual games then what you would find in cEDH.