Why is there no job description which requires Monad knowledge?

It's because it reduces the candidate pool too much, and even jobs where you will definitely use monads like some devs at Jane Street they will accept candidates who don't even know functional programming yet.

Because monad knowledge has negative value. Introduce the idea that "a monad is like a burrito" and a software team will accomplish nothing for a year.

Knowing what a monad is -as in, really knowing - is not a requirement to properly use them. It would be pure noise in a job description.

Because it would be more costly to wait for a unicorn than it would be to just hire someone and teach them.