My issue is that there are only so many times you can go thru the process of learning a new language. It's monotonous. You have to learn a new way to create code without really advancing your knowledge. You can learn to create a loop in 10 different languages and still not advance your programming abilities. You just know 10 different ways to do the same thing. Not only that, your time is finite. The time you take to recreate the wheel could have been better used somewhere else. Learning a new language is like spinning your wheels rather than moving forward.
It isn’t difficult to learn a new programming language, especially if based on a known paradigm. What’s difficult is sorting out the multitude of tiny differences in their implementations and libraries.
(This assumes a relatively elegant language, not a bag-of-tricks one)