They are dehumanizing because of the relationship to the work. It is of no value to the worker. It is meaningless. And that is dehumanizing.

Digging holes in a random field for 8 hours a day for minimum wage to make someone else richer is rather dehumanizing. When it's digging holes for a fence in your yard, or for a community barn-raising -- the exact same work -- somehow it's less dehumanizing.