If by "anything" in the phrase "hadn't learned anything in 20 years," you mean things like legal principles, rather than specific new precedents and laws, I suspect a lawyer probably doesn't have to learn much after they clear the bar exam. If any lawyers read this, I'd love to be proven wrong. :)
I think you're underestimating how fast the law changes. In a lot of practice areas (I work in tax), there can be substantial changes every few years, and in between, daily work changes as everyone converges on optimal strategies, and then the law changes again. Then there's new software constantly, which you generally need to be familiar with or get left behind. Then there are the gimmicks of the day that your clients find on the internet that you need to be familiar with or risk looking like you don't keep up with best practices (even if the new stuff isn't anywhere near a best practice, or is only applicable to Fortune 500 cos.).
I would go so far as to say that a lawyer who just recently passed the bar exam is substantially worse at the practice of law than a paralegal with 20 years of experience. The legal principles learned in order to pass the bar exam are akin to... basic algorithms (maybe?) for a software engineer. They're important, but they're also not really what the job is on a daily basis.
