Why U.S. warships retained large crews

Did abundant manpower keep the U.S. Navy from pursuing extensive automation until the twenty-first century?

Yes in large part, reinforced by an institutional tendency to solve demanding shipboard tasks by assigning more people.

A large available workforce reduced the pressure to eliminate shipboard billets. An American institutional habit of solving problems by assigning more people reinforced that effect. Automation therefore faced a weaker incentive than it would in a navy forced to extract the same capability from scarce crews.