What professors at research universities do:
1. Mentor PhD students (and often also MS and BS students)
2. Write papers and edit student papers
3. Write grant proposals
4. Deal with a lot of administrative issues
5. University service committees - some are on multiple some are on none (admissions, COVID response, undergraduate program, graduate program, etc.)
6. Review papers from others
7. Teach one or more courses
8. Develop new courses (takes 1-2 days to make a good lecture)
9. Update lecture materials (I spend probably 2 hours per lecture for courses I've already taught)
10. Meetings
11. Conference/Workshop organization
12. MANY miscellaneous duties
13. Budget management
14. Prepare talks
15. Serving on MS/PhD proposal/defense committees for students not in your lab
16. Staying on top of the scientific literature
17. Making homework assignments (some can delegate to TAs)
18. Grading (some can delegate)
19. Write letters of recommendation (I write around 10-15 per year)
Of course you can get this load down by learning to say "No" in some cases. But a professor during their first couple years when they start the job can easily expect to put in 60-80 hours per week. It gets a lot easier after that.
1st year: we had a 1 hour meeting every week
2nd year we had a 1 hour meeting every 2 weeks
3rd year (1t hslf) we had a 1 hour meeting every 2 weeks
3d year 2nd half onwards we had a 1 hour meeting if I emailed them and asked them for it
If I needed more time I could always knock on their door. And oftentimes we went to have lunch for informal meetings.
Now, i always felt my supervisors were absolutely amazing. But i quickly understood that to complete my PhD, i needed to scratch with my own hands and that my sups were there only to guide me.
All in all my experience was great. Nevertheless after finishing and doing 4 years as a postdoctoral i decided the "publish or perish" mentality was not for me, and moved to industry.
Also they are your advisors, not teachers. At PhD level, you are supposed to make a meaningful contribution to the discipline on your own.
Mentoring by more senior folks in the lab certainly helps everyone (https://www.pnas.org/content/116/42/20910), but it's barely measured or rewarded for PIs and not at all for postdocs. Changing this would be an easy fix though--ask for a reference from a junior colleague when hiring.
More generally, there's a huge disconnect and sudden between the skills needed to get a faculty job (i.e., publishing a high profile papers as an individual contributor) and the skills needed to do well at it (e.g., management). Everyone is all antsy to shorten postdocs, but I'd actually be in favor of making the career path more gradual with so people get some mentored management experience.