I agree with you: learning a new language, sometimes in the heat of the moment without even knowing the language existed before, is a thing skilled software developers are able to do remarkably quickly, even if they haven't specially gone out of their way to train for that skill, as, in the real world, you end up dealing with so many different languages--as a C#/Java developer you probably also have had to work with SQL, either bash or PowerShell, can't really avoid JavaScript, likely had to drop to C/C++ at some point to build a way to access a "legacy" component--that it is very difficult to avoid having the right generalizations happen accidentally in your brain.

But, I work with a lot of college students who are just beginning to learn software development, and the realization that that kind of generalization is possible is not obvious; and a lot of people who are only a few years into their career--but maybe think they are pretty damned good at it (I look back at myself when I had a mere 10 years of actual-real-world experience and think just how much I still had to learn to become who I am today... I am thankful I worked with some gods and always knew I had a long way to go)--not only have a hard time learning a new language (I remember when I was in high school trying to learn Pascal after I had spent years coding in Visual Basic and even getting paid to do so, and thinking I knew C... I did not ;P) but internalize how daunting it is and then go really far out of their way to avoid learning new languages (which, thankfully, I could not as high schools at the time were ping through a big upset in curriculums and so we learned a new language every year for a while and I got recruited to help my AP Computer Science teacher as he also was having to adjust way too quickly through Pascal, C/C++, and Java/JavaScript).

And it is actually quite remarkable how well people manage to do at this, using IDEs to avoid command lines and ORM technology and DSL compilers to avoid having to use SQL or other languages, they glom onto whatever ridiculous interop mechanism they can that prevents them from learning about low-level languages... and I think it really does hurt them, and I remember running into some software developers who did so well that they never managed to get to the right generalizations and they are now old and can still only program in one language (the same way I have a really really really hard time speaking in any language other than English); but like, most developers, no matter how hard they try, eventually do get enough exposure to enough at least slightly-different languages to get this skill (which is, when I get hired to teach courses, what I try to accelerate: Comparative Machine Language Morphology ;P).

But like, due to us all living in the Eternal September--and I do not just mean online: I was shocked recently when I learned how many CS students there are at most of the other Universities (it turns out the one I still live at is an extreme minority with only a few percent of the population being Computer Science and only half the students being engineering at all)--not only are we going to end up interacting directly with a number of people who actually do find learning new languages hard (and think it must fundamentally be hard for everyone forever), but I think we are going to run into a lot of managers on the front lines of trying to build businesses out of cogs (a practice I dislike on a number of grounds, but fully admit, if you can pull it off, is probably an extremely powerful and efficient way to do industrial/corporate engineering) who are also extremely resigned to the notion that the early-stage people they hire are oft incapable of learning new languages quickly enough to truly be a usable cog, as like... do they really even want to hire someone who has as much experience as you must (as the way you worded your comment implies you've been coding a lot longer than the mere 5 years you have been coding in Java/C# ;P).