Many companies have a formal mentorship program but it is often in name only (at least in my experience at 3 companies).
In a formalized apprentice program, a worker can feel okay about not knowing things and asking the person for whom they are apprenticing. Despite formal mentors, I believe many junior devs do not have a person that they feel comfortable not knowing things. Moreover, plenty of senior engineers do not make good mentors (either don't want to or don't have the strongest of communication and mentoring skills).
You'll have a crop of engineers with 0-2 years experience that don't feel comfortable not knowing something because they feel it can be a sign of technical weakness. Perfectly valid questions go unasked and a young engineer may spend 3-4 days trying to answer something that his mentor could help him/her with in 15 minutes.
Great knowledge isn't transferred optimally in this scenario.