Because we've only got 120 pages (maximum) to tell that story. Because no one cares about the "Hello, how are you?" "I'm fine, how are you?" dialogue. Because every scene is a miniature story in itself, and you always start a story as far in as possible.

Our stories capture "a", possibly "the", most important story of our character's lives. Dialogue and actions are the tools we have to tell their story. Any dialogue that does not 'tell the story' - or aid the telling of the story in a significant way - isn't just space filler. It detracts from the story, it lessens the impact.