According to a study described in the Stanford Social Innovation Review:

“The primary difference between volunteers and non-volunteers, when measureing what they do with their time, is the amount of television they watch. People who do not volunteer watch undreds of hours of additional TV a year compared to people who do volunteer.
It’s not that people dont have nough time to volunteer. People do not volunteer because nonprofits do not provide them with volunteer opportunities that interest them enough to pull them away from their television sets.”

The rest of the piece outlines the ways that non-profits mismanage volunteers and how they could improve. Many charities make heavy use of volunteers but few organize or use them well, the authors argue. In all, they said more than one third of those who volunteer don’t do so the next year, an indication they weren’t captivated — a loss of $38 billion in lost labor.