Knowledge transfer requires practice. They haven't built one for 50 years, so there hasn't been knowledge transfer.

I'm sure that NASA has all the blueprints filed away somewhere, but the reason we don't have a Saturn V factory running today is not because NASA forgot how to make them. Instead, it's because of cost. Between the Apollo 13 disaster, the Vietnam War, and maybe some other factors, public interest and approval of continued Moon exploration wanted, and Congress revoked the planned funding for Apollo 18 through 20, opting instead to focus on programs like the Space Shuttle.

Interestingly enough, the leftover Saturn V hardware was put to good use by launching Skylab missions and the Apollo-Soyuz Test Project, both of which turned out to be valuable steps in the US space program. So as much as it pains me to say, it may have been a good thing that the last three moon missions were canceled.

The last moon landing was 1972. That's 51 years ago. They were all part of the same Saturn programme, production actually ceased 4 years earlier in 1968. At the time it was the most complex machine ever built.

>There must have been a knowledge transfer for such an important feet of engineering.

You've never heard of the situation where no one knows how a business critical piece of software works? "Bob wrote it 20 years ago but he died last year". This kind of stuff happens all the time in the real world. If no one is paying for that knowledge and supply chain to be maintained it will atrophy and dissapear. That's why the US army is still buying tanks even though they have thousands in storage, they need a company to maintain the supply chain and institutional knowledge to build tanks. Are you imagining generations of engineers being tasked with knowing how to build a rocket with 1960's technology with parts from suppliers that no longer exist despite no one having any intention to ever buy such a rocket again and no one paying for the maintenance of that ability?

Because money. Why is this hard to understand?

NASA didn't have the budget to continue to operate Saturn V and also build Shuttle.

The president and NASA leadership wanted Shuttle. So the last Saturn V were put into storage.

To claim they don't exist is stupid, you can go see them:

https://ids.si.edu/ids/deliveryService?max_w=800&id=NASM-A19...

Yes, lots of those design top documents still exist. But not every supplier and sub-sub-sub supplier did the same thing. Most of those companies don't exist anymore or were bought and bought again.

NASA never wanted to build a Saturn V again. So they archived all the plans.

You seem to believe that they put some kind of plan in place to keep the Saturn V so they could bust them out again. This is simply not the case. As far as they were concern Saturn V was over and Shuttle was the future. Archiving everything was the only thing they did.

Of course we could do a huge effort and recreate the Saturn V program. And the lots of documentation that exist would help. But anybody who has recreated old things, knows that plans are not perfect. Doing something like that would simply not be worth.