Because they may have studied history... There are countless examples of eras of lost technology due to a stumble in society. Where those societies were never able to recover the lost "secrets" of the past. Ultimately, yes, humans can rediscover/reinvent how to do things we know are possible. But it is a very real and understandable concern that we could build a society that slowly crumbles without the ability to relearn the way to maintain the systems it relies upon, fast enough to stop it from continued degradation.

Like, yeah, you have the resources right now to boot strap your knowledge of most coding languages. But that is predicated on so many previous skills learn through out your life, adulthood and childhood. Many of which we take for granted. And ultimately AI/LLM's aren't just affecting developers, they are infecting all strata of education. So it is quite possible that we build a society that is entirely dependent on these LLM's to function, because we have offloaded the knowledge from societies collective mind... And getting it back is not as simple as sitting down with a book.

And we're still here right? We have more books and knowledge and capabilities than ever. Despite theoretically losing knowledge along the way, we're okay (mostly).

Society can replace the systems it relies on. The replacement might not be the best, but it'll probably handle things until we can reinvent a newer, better system. It probably won't be easy, but you can't convince me that humanity suddenly cannot adapt and fix problems right in front of them. How long does history have us doing that?

These are extraordinary claims that all of society will just become dumb and not be able to do any of this. History is also littered with people fretting about the next generation not being smart enough or whatever, and those fears rhyme pretty closely with what we're talking about here.