This is a very simplistic view of the situation. It doesn't usually work like that in the real world.
In the company I left few years ago I was a lead developer in a department which started with me alone, then at times grew up to four developers and shrank to just two.
Here's what it looked like: we were almost always advertising a position in my department, and about once a week I had to meet another Python developer with X years of experience in infra, who was barely literate... No, really, it was that bad.
After each such interview, I had to go back to my boss and decide whether we should give this guy / girl a chance, or should we spend more time and money looking for more candidates. I tried both strategies. I tried to hold out until we find a better candidate, and we were simply wasting time running in circles. I tried to advocate hiring someone simply on a basis that they looked promising. I had to promise my boss to bring them up to speed, and that was a year of misery of dealing with someone who made such a painstakingly slow progress, being such a huge time sink that in the end I had to abandon that person to have at least something done.
Infra projects, especially in large-scale products tend to be this way. Outside, theres' a huge pool of "Python programmers", but most of them are really "Django programmers", they have no idea how computers work, and will require years of mentorship to obtain that knowledge. Few will see that as valuable as it's not a common enough requirement. Few will have the determination or passion to get to the point where they are confident and independent that they may continue their education on their own.
On the other hand, those who gained such experience don't like infra jobs for the very reason that they end up recruiting novice programmers and leave for more system-like jobs. And this dynamic affects every department both at a macro and at a micro level: people either learn how to be good and leave, or they never do and get replaced by the next batch.
In the end, it's fairly typical that infra department will be "carried" by a single employee who's spend ages working there + a number of younglings who come and go never contributing anything of note. I wouldn't say it's a management problem. Infra isn't the only department which sees that kind of dynamics. QA is pretty much the same way. Front-end used to be more like that, but I don't know what the situation is today. It's caused by the pathological relationship which reinforces itself with time that prevents expertise from growing both locally in an org and generally in the field.