Cultures and countries under siege often respond to these threats by expanding into warfare and empires. A threat from outside, real or imagined, is supposed to unify the defenders.

Missionaries are emissaries of the Mormon empire. As long as these young people are engaged fighting for Mormonism, they are less likely to critically examine it. There is an old adage that "you love what you work for." Whether or not it was worth the work, you have invested your precious time and credibility and other resources in this enterprise, so you are reluctant to change. It's like an abusive marriage that's tolerated because of the horror and "shame" of divorce.

I can testify that this is true.