I can get cooking with a programming language I’ve never seen in my life in a matter of days because most programming languages are either very similar to each other or they’re similar to some paradigm most of us already know like functional programming.
The companies looking for 5 or 10 years of experience with a language aren't looking for someone to "get cooking" and learn as they go. It's true that most experienced developers can quickly pick up the syntax of a new language and start writing code quickly, but every language has its nuances and pitfalls and you're not going to know them all after a month or two of writing code.
At a certain level of experience, you do have an idea of what kinds of things can be nuances and pitfalls, though. And for certain team/codebase sizes, the effort required to learn and adapt to the specifics of the in-house systems will almost certainly dwarf the pitfalls of the general parts of the language/platform/stack.
There's some shortcomings of the algo-oriented interviewing practices FAANGs are famous for, but IMO one of the things they get very right is that language/stack knowledge is considered less interesting compared to general problem solving skills (and I say that as someone who failed to get an offer on his last round of trying).
The companies looking for 5 or 10 years of experience with a language aren't looking for someone to "get cooking" and learn as they go. It's true that most experienced developers can quickly pick up the syntax of a new language and start writing code quickly, but every language has its nuances and pitfalls and you're not going to know them all after a month or two of writing code.