Multiple users on this site have stated that only around 1 in 10 (or maybe an even smaller ratio) PhD students are able to successfully continue in academia, beginning with an Assistant Professor position and working their way up towards tenure. (I am unable to find links for this, but I have read it in at least 5 different answers/comments on this site.)
For the purposes of this question, I am ignoring those who actively want to pursue a career in industry, and am focusing on those PhD students who wish to go into an academic career and fail to successfully do so. I wish to know why students fail to make the transition, and what exactly is the hardest part of this.
For instance, is it:
- The transition from a PhD position to a postdoctoral position: A postdoc has to work much more independently than a PhD student, and maybe people struggle with this.
- The next transition, from a postdoctoral position to an assistant professor position, or,
- Simply making a bigger impact on one's research field of interest, after securing a tenure-track position, and hence moving away from academia into industry.
In which part do newly graduated PhD students, who actually want to go into academia, "fall by the wayside"? Or, which of these transitions is the hardest to make, resulting in students not making the transition to a full academic career?