Where is the joy of learning a new language if not for learning new concepts? It just does not make sense to me. Learning a new standard library and build tools and all that stuff? Is that much fun?

People want to learn a new language for reasons: perhaps they want to learn new concepts, perhaps they want a language that's faster, perhaps they want a more mature ecosystem, perhaps they want to increase their employability. There aren't people who "want to learn a new language" as their ultimate end goal.