Because buying your own lies about the strength of your army makes you complacent.

There's a reason why dictators have a terrible war time track record.

You tend to distrust the propaganda of the other side. You are not quite as distrustful of the propaganda of your own side. If it is clever enough not to appear as a cheerleader like this article, you may barely notice it cherrypicking the benefits of a story.

Obligatory: "Are we the baddies?"-sketch illustrates the concept very nicely.