“We haven’t forgotten anything about building rockets...”
Most of the folks who did that work are long retired and/or passed on. Sometimes institutional knowledge about what works and what doesn’t is as important as book learning. Most of the engineers at NASA today haven’t a clue. What they have been told is fast and cheap. Buy off the shelf. Don’t design anything new.
We may not be able to replicate the work done
Replicate?
We wouldn't want to do that ... the underlying technology of metallurgy and other materials (for example) has changed dramatically for the better since then. If "we" really wanted to go to the moon again, we'd want a XXI Century rocket (scaled up from current designs, just like the Saturn V was an extrapolation from previous designs) ... we would NOT want to just rebuild the Saturn V.
Your point about lost institutional knowledge is valid ... and a good reason for not just dropping things like NASA just dropped big rockets in the late 1970s ... but it's not a mission-killer. Just a major annoyance.
For example, I gather they're having problems with Ares where the vibrations caused by the large solid boosters are resulting in a vibration problem, resulting in a requirement for more bracing, thus more weight, thus larger solid boosters, thus more vibration, and so on down the line.