It bewilders management, because there's a very significant overhead involved in making sure an employee is properly synced on what needs to be done, making sure they are content and productive, and managing the administrative logistics around them. Even disregarding the work of management, in a flat team the communication overhead that each member adds can also be significant and non-linear.
Generally, adding people adds a lot of complexity and inefficiency to an organization, and if you can do something without more people that's usually a lot better. It depends on the role of course, but in many jobs now an employee that is not fully dedicated can be a net-negative. The same can be said of employees that are not very experienced or competent.
This is why there's a significant crisis in early-career employment. More generally, it's also why we have a large fraction of population feeling like they cannot get a decent job, while many companies are simultaneously struggling to find the employees they actually need for a reasonable salary.