A World Without Engineering Managers?
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The answer of course being: "because I couldn't manage myself, and if I did I'd probably be even worse than them".
Not having a manager is an option every single dev has, at all times. They're in charge of that decision.
Most managers perform poorly. That's a trite fact that anyone with a few years of career experience under their belt will know full well.
But they're not putting their hand up for the role, and they're also not putting their hand up for the job of managing managers (i.e. starting a successful concern) so they can make their company the exception.
Instead they want to flippantly pretend that code by itself is valuable, without an apparatus to connecting it to the market that ultimately pays their rent.
The answer is always the same: find better managers then. Maybe a new better employer that trains their managers well and keeps the manager-reports ratio high. Maybe making yourself the manager.
Self-indulgence isn't the way.