Earlier quoted context omitted.
A fresh postdoc able to secure an academic job joins as an "Assistant Professor". Fresh Assistant Professors usually cannot recruit PhD students, have limited funds to establish their lab, and no job security. It takes several grades and 15+ years to reach the position of "Professor", which comes with tenure.
Who told you assistant profs can’t recruit PhD students?
I think there's also a practical problem in attracting students to a new lab. Another Assistant Prof I'd interviewed with at a premier research institute in New York had established her lab over 3 years ago, and still hadn't managed to recruit either a PhD student or any postdocs. She may be an outlier, of course.