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.
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.