Because we're social creatures. We're desperate to foster and measure social consensus. That's why we have so many meetings. We want to see the agreement / concession / enthusiasm / disagreement on people's faces, and then use the social forum to cow them into assenting.
Because this is a base intuition and inclination, we employ this clumsily even in cases where it's not really the right tool for the job.
This is the core of it. So much of business, engineering and product design is about navigating a network graph of people to discover requirements, coordinate expertise and make things happen in the real world.
I know many engineers would prefer to just focus on engineering, but creating something meaningful for people means working with and through people. Even the most hands-off, self-service SaaS products depend on this.
Take an extreme case like Minecraft, where a single person behind a computer created a billion-dollar company. I'd argue that even here a significant part of that success came from Notch's very active involvement in the community.