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.