It's more like we've lost the engineering. The technology is all still there but now greatly improved. From welding techniques to computer components the whole exercise in in manufacturing would be a huge undertaking to rebuild because we aren't manufacturing any of those old technologies anymore so that would be a problem. Or you have the problem of re-engineering the whole rocket with modern components and manufacturing techniques.

We can build medieval castles all day long with concrete and steel, but if you want an actual stone medieval castle, we don't know how to do it.