I've been there, in small cohesive high functioning teams there is a sense that a manager isn't really needed, and to a degree that's true while things are going well, right up until shit inevitably hits the fan in one of these scenarios.

A medium or larger business also usually wants planning and reporting and analysis that devs usually hate to create, present, or maintain because it eventually takes a big chunk of time away from developing. It doesn't make them bad devs but the management aspect gets neglected and the business suffers as a consequence, leading to eventual layoffs or shuttering. I think the amount of management needed is a wide open discussion, but questioning if it's needed at all is naive.