Most of the time the professors spend 0 time specifically on a student (PhDs excluded) as postdocs/PhD students mark coursework and even (in some institutions) exams
lol you think professors spend time on their phd students? they have postdocs for that...
edit: a typical research lab functions exactly like any other organization in the world: hierarchically. a chief executive/leadership role/head (the prof), his/her direct reports (post docs), their direct reports (phd students), and their direct reports (undergrads). if a phd student is being directly advised by the prof that's simply because the prof currently has no postdocs not because they don't desire some.
you're wrong - eg in my uni (a "top tier" uni) thesis advisor could be a RAP (research associate professor) which was not a full nor tenure track prof and thus not much more than a postdoc.
Mine doesn't even have many postdocs (if any) as far as I'm aware. Similarly, while sometimes promising PhDs might be assigned to handle undergrad researchers, most of us worked directly with the professors back then.