We don't learn; we race into tar pits.
Oh sure, we naturally learn at a very rapid pace right up until the moment we can ship production code at that new job. I submit that our natural state after that is a plateau, we all but stop naturally getting better. We're in a hurry, right? We've got production code to ship so when we're under the gun we keep reaching for the first thing that worked. Ever hear "the same year of experience 5 years in a row?" Yeah, that.
I believe that this is why the common advice of learning a new programming language every year came into vogue. It gives us a place to zero out our proficiency so we can race into new and exciting tar pits.
I also think that learning a language that we're never going to put into production is a highly inefficient way to get better. We can stay at that job, using the same language, and through dedicated practice become terrifyingly good.
Learning on the job might not be a lie but it sure as hell isn't the default. We've got to opt into it.