Eventually seniors leave. You need people to replace them. You don't want to pay the highest possible price. You want people who know your system already. Hire juniors and train them up. This isn't hard.
Or it can be near impossible. Large orgs can have dedicated onboarding programs for junior developers, this can work quite well. Small or medium orgs can have one or a few senior developers who are passionate about mentoring. This can also work well, but is unusual.
What is much more common is for new hires to encounter a nearly impenetrable wall of “figure it out, don’t bother me”. I have experienced both, and I’m sad to relate I’ve been a wall at times. I found I could only explain a stack trace n times to the same person before I personally noped out of training juniors.
It made me really start to learn on my own and figure out how go from 0 on many different skills. So personally I really think it’s about the junior and how much they’ve figured out their learning style, and being “a wall” like this can sometimes force them to learn by reading through all of your old code. Inadvertently leading by example is still really good
In short don’t sweat it