You have a point here.
However, it is important to understand that American manufacturing industry is sufficient to develop all components of an aircraft at home.
Industry is the key word here because a number of domestic companies might be involved in the development of an advanced aircraft and a few sub-components might be acquired from subsidiaries of foreign vendors but these components are also manufactured in the US. This is how things are for the design of aircraft in the US on average.
Here are the details of components of F-22 Raptor aircraft and their respective suppliers:
http://www.airframer.com/aircraft_detail.html?model=F-22_Raptor
Now, exceptions are also there such as F-35 project. However, American expertise have been invaluable for it.
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Chinese manufacturing industry comprises of a large number of foreign vendors and domestic firms are select-few. This implies that scores of sub-components and even components would be sourced from foreign vendors. For example, Russian engines are commonly used in Chinese aircraft. Similarly, Russian input is often involved in design of other components and even avionics. Though Chinese are learning.
However, it is too early to expect Chinese to match the expertise of Americans in different areas. Otherwise, China would be developing every component at home and not buy stuff from others.