It's usually intentional so they're not going to show more information to reveal that their narrative is weak or wrong. It's up to the reader to think critically because journalists are constantly trying to fool them with true facts presented in a misleading way.

Readers are guilty too - they like to see wiggly lines wiggle. Nobody wants to see a graph that's just a horizontal line or shaded band. We want to peer harder at it to tease out any sort of signal that can tell our emotional brains "good" or "bad".