Branko Milanovic (HT Mark Thoma) misses the point about the usefulness of the concept "human capital". To explain the point, let me talk about "land capital" instead.
Raw land, in it's natural state, often isn't very productive. Before you can grow wheat on it, you usually need to clear it, or drain it, or fertilise it. There's an up-front investment, which is costly in the "opportunity cost" sense, because it uses resources that could have been used elsewhere to produce something else.
What we call "land" is as much capital as land. The rents on "land" are as much a return to capital as they are a return to raw land.
Some land needs very little investment to make it productive; other land requires a lot. Some land gives a high return on investment; other land gives a low return. It's all different.