That it's relative share in GDP is down during that time means that other sectors were growing even faster (think Google, Netflix and so on, so services instead of things). That the service sector gains in relative importance is actually a sign of an advanced economy, every modern economy looks like that, not just the US.
Basically if you ignore computers, there was zero growth in manufacturing sector. If you take computers into account but ignore processing power increase, there was zero growth. So pretty much the only driving force in the manufacturing sector for the last 20 years was Intel, AMD, Nvidia. This is even with the increased productivity per person in manufacturing sector (so there was also massive reduction in employment).
It is highly concerning and the numbers were very much cooked up.
https://www.macrotrends.net/global-metrics/countries/USA/uni...