You know how you've take a course before where the professor was just surprisingly awful at teaching? These professors are often some of the most knowledgeable people in a subfield of the subject you are taking, yet their teaching ability is severely lacking and you have to scramble to learn the material some other way (or just never learn it).
During a PhD, there is a decent chance that your adviser is similarly a bad manager. Unfortunately, having a bad manager for 5-7 years of your life can be a fairly awful experience. You will work with someone who you, on the one hand, look up to, but on the other hand, who seems to not care at all about your mental health, your possible career desires outside of academia, your work/life balance, or the exact reason why this week was a rough week for research in your (human) life.
I have a lot of other thoughts on the matter, but I thought I'd try to keep this post more concise =).
I wouldn't call that being a bad manager, but rather being an asshole.
As a professor, I often think that one of my biggest weaknesses is indeed management skills. After all, we suddenly find ourselves having to manage people without any training in the matter, and when our true call is typically science, not management.
But at least I'm not an asshole.