Junior engineers aren't toddlers. Hopefully they're a lot more competent. Nobody hires toddlers to do work.
Here's my analogy: if you want to learn how to ride a bike, you've got to take off the training wheels, and sometimes that means falling over.
Not every problem is going to take junior engineers days to solve for themselves. I was stating an upper bound of how much rope to give them, not an average.
Besides, what happens when the senior engineer leaves the company? Sometimes "institutional knowledge" walks out of the institution.
It sounds like you are trying to address issues during mentoring that should have been addressed during the hiring process. In my experience, all the juniors I've worked with come in with the idea that they can solve every single issue themselves, and part of my job is to show them how wasteful that is, that a simple question can save the whole team days if not weeks worth of time.
If someone walked through the door who is not enthusiastic about solving technical issues themselves...your hiring process needs work.
Sure, reinventing the wheel every time is a valuable experience, but it's not efficient to relearn how to do things every time.
This is like letting toddlers struggle to learn how to tie shoes themselves, sure they will eventually figure it out, but it's better to just teach people have to do the basics, and let people bang their heads against hard problems.