Jack of Hearts Discussion

So I was rewatching aib this past weekend and I noticed a slight plot hole. Since all of the face card game runners are predetermined (whoever decided to stay in the borderlands), why did people not inherently trust each other. For example, the man and the woman who were partnered. Why was she so inclined to trust a random person over someone she's known for a while now, who probably went into the borderlands with her. If the jack is already predetermined, (the characters know this from other face card games), if you go in with a friend or someone who joined the borderlands with you, what do you have to worry about?