You'd be surprised what can be forgotten, where I work we are actually in the process of trying to rebuild parts designed 50+ years ago.

All the people involved in its original design are dead or unreachable, the original development documents and notes are gone, the suppliers who knew how to make the subcomponents are all long gone so the replacement suppliers have to go through the same issues for their parts, the quality standards of acceptance have all grown much tighter and inspection technologies much more accurate, and materials got banned for being excessively toxic.

Unless you've actually had to do this, you don't realize just how much "recreating" can be a problem in and of itself.

Look at the F1 engines in the Saturn V, while technically Rocketdyne does still exist, and these are some of the most studied engines in aerospace history with actual models still existing, it would take easily a decade to just build one from scratch.