Is there a word or phrase for 'shared accountability that leads to collective inaction'?
Solution 1:
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.
Solution 2:
The proverb I've heard for this is everybody's business is nobody's business. See here for more.
Solution 3:
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.