Depending on the business situation, that would be a complete disaster. You don’t want 10 people working on a 5 man job, when 2 more jobs are both languishing, and time critical. A good manager can make the calls about which work can wait, and which needs to take priority, and how to balance things. They can also work as primary points of contact between different teams and companies. This keeps information flow clear and stops people being left out of the loop.
While all of these can be done without a manager, the efficiency plummets as the number of people grow. By the time you’re dealing with large, inter-company politics, a manager is critical to take load off the workers. I don’t want to spend 80% of my time keeping people informed, when I can spend 2% and get on with my actual job.