Well, it's not just because of outsourcing from big companies, at least according to this research paper by a couple of economists.
What Accounts for the Rising Sophistication of China's Exports?
> by Zhi Wang, Shang-Jin Wei - #13771 (ITI)
> Chinese exports have become increasingly sophisticated. This has
> generated anxiety in developed countries as competitive pressure may
> increasingly be felt outside labor-intensive industries. Using
> product-level data on exports from different cities within China, this
> paper investigates the contributing factors to China's rising export
> sophistication. Somewhat surprisingly, neither processing trade nor
> foreign invested firms are found to play an important role in
> generating the increased overlap between Chinaâ?Ts export structure
> and that of high-income countries. Instead, improvement in human
> capital and government policies in the form of tax-favored high-tech
> zones appear to be the key to the country's evolving export structure.
> On the other hand, processing trade, foreign invested firms, and
> government-sponsored high-tech zones all have contributed
> significantly to raising the unit values of Chinese exports within a
> given product category.