This is because they massively overinvested in engineers. It sounds crazy to an american but there is such a thing as building to much.
The railways are a perfect example, when they built the first railway from north to south it was a
massive productivity boost to the entire nation. Now instead of a 3 day car ride you can take a 6 hour train ride.
Then eventually they build a second rail line parallel to that one 20 miles over. This is convenient, but not as big a deal as the first one. The people 20 miles west of the first rail used to have to drive to it, then take it, now they can just hop right on the one in their town but this is only saving them a 30 minute drive rather than a 3 day one like the initial rail. It's diminishing returns.
This applies to factories, powerlines, plants, housing, and all infrastructure they've made. Eventually you run out of things worth building which is a problem when more than half your population only knows how to build shit.
Their plan with Belt and Road was to basically export these engineers to tap the incredible wholesome unrecognized potential of the third world and we all know how that's going.
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