When a group as opposed to an individual is responsible for a job or task, and as a result the task does not get performed because everyone assumes that someone else will do it.
Is there a word or phrase to describe this?
The term I've heard in the workplace is diffusion of responsibility:
Diffusion of responsibility is a tendency of individuals in large groups to avoid taking action to correct a problem because they assume someone else will.
The proverb I've heard for this is everybody's business is nobody's business. See here for more.
Apparently not as common as I'd thought (less than 100 written instances using it as a noun phrase), the first term that came to mind was dissipated responsibility.
This is a form of diffusion of responsibility.