I've put together two futons in two days. One of them was an old wooden one, and the other was metal with a bunk bed on top.
Both of these projects took only a few minutes to complete. This included figuring ut how the pieces fit together, putting them in place, sticking in the bolts, and tightening everything.
Three people times five minutes is fifteen minutes of combined time. But if there was only one of us, it would have taken far more than fifteen minutes. Holding the pieces in place took both hands, so bolting things in would have been nigh impossible.
The same is true of writing. Sometimes, writing is a three-man job. One person is the primary writer. The second person is a developmental editor, who aids in crafting and honing the story. And the third is the copy editor, who proofreads and polishes the lines and passages of the written words.
Sure, one person can do all those jobs, but it just isn't the same. For one thing, the quality of the results may not be as high. For another thing, it may take far more than three times the time to do those three jobs by yourself.