Knowing how these layers work as concepts is a lot more important than deep expertise with a particular implementation.
From the perspective of a new hire, you'll spend easily as much time learning their idioms, mutations of the platforms, unique glue code and business logic as you will learning the platform.
unless that is a trivial amount of time, which it generally isn't (and obviously can't be if you are expected to learn a new platform in the same amount of time to a level of reasonable proficiency), then the amount of time that you would devote to the learning it is probably not that worthwhile for the company to let you lose on the platform you don't know.
Of course my perspective is as a consultant, so if I'm brought in for 6 months and I do not know part of their stack it is just not worthwhile for me to learn that part of their stack, they generally want me to be productive on the parts of the stack I do know. If I was going to be at a place for some years maybe the motivations would change, but bigger places tend to optimize and specialize their developers and not want people to be "full-stack".