You're not wrong at the high level, but learning a particular language to the point of fluency is still an investment if you're interested in being productive while writing it. Sure, once you have the experience, reading most new languages is easy -- but learning the idioms and particular syntactic choices of a new language to that point that you can reach for them without having to constantly search or consult references isn't something that's instant. There's also a mental cost to how many of those languages you can have "ready to go" in your brain at any given time, and for languages that are evolving -- most of them, at this point, honestly, even old languages like C++ -- there's a cost to keeping up with the language as it changes.

All of that is investment, and not all languages pay off that investment.