The answer is simple: Those are typically optional topics in a CS curriculum. You might as well ask why an undergrad doesn't know compression algorithms, compilers, robotics, software reverse-engineering, cryptography, computer vision... All of the above are optional and will only be taken by those who are interested in them.
The more important question is: Are you explicitly mentioning databases, concurrency and networks in your posting? If not, then it explains why candidates are not filtering themselves out.
At some level, we had SQL and network programming in my degree, but only for that level that we knew it exists and how to do it in very basics.
We did not have concurrent programming at all as mandatory course.
The most of your deeper expertise comes from the project work here, and depending on which project you end up or choose, they might or might not include above.
Software is not unique in that it has a high degree of specialization that cannot be touched on in a general education. Probably 90% of skills/knowledge in my non-software engineering career was not taught in my program and generously 5-10% more may have been taught in the most rigorous programs.
Not sure where you are based, but in the US I know a lot of schools shoot for the ABET accrediting guidelines[0]. They directly call out the areas you're asking about:
Exposure to computer architecture and organization, information management, networking and communication, operating systems, and parallel and distributed computing.
But they only require "exposure" to these concepts whereas algorithms, theory, programming languages, etc are required to be "substantial." Which matches with your perceptions:
It seems most are exposed to them partially through project work but without the base knowledge.
The more important question is: Are you explicitly mentioning databases, concurrency and networks in your posting? If not, then it explains why candidates are not filtering themselves out.