We stopped doing serious space development after Apollo and lost a ton of institutional knowledge between then and when SpaceX started picking up where they left off.
Documentation and old drawings, often lacking implementation details, can only take you so far
There's no big secret, if we do a thing a lot we will be able to do it consistently and reliably. Boeing has not done a lot of spacecraft design and manufacturing recently. They've spent a bunch of "time" on it, but haven't actually produced much.
Fortunately other companies, besides just SpaceX, are building lots of spacecraft.
Documentation and old drawings, often lacking implementation details, can only take you so far
There's no big secret, if we do a thing a lot we will be able to do it consistently and reliably. Boeing has not done a lot of spacecraft design and manufacturing recently. They've spent a bunch of "time" on it, but haven't actually produced much.
Fortunately other companies, besides just SpaceX, are building lots of spacecraft.