There are many primary care doctors in the US that are at least as good as foreign doctors. However, there are many things that factor into your observation. The first is that many highly skilled physicians choose to sub-specialize because it is both more academically stimulating and more lucrative (I am a sub-sub-specialized physician). For better or worse, the field is geared toward sub-specialization. This dilutes the number of quality primary care physicians. It also puts a ton of time pressure on primary physicians, since we have fewer primary physicians per capita than many other nations. They simply don't have time to perform thorough exams so they rely on referrals and testing.
Also, there are expectations by patients placed on primary physicians for tests and drugs. Primary physicians, under financial and practice pressure to keep things moving, tend to get into a pattern of giving the patients what they want to keep things moving. This tends to create bad habits. I will say that this happens much less at good academic institutions where there are somewhat less financial pressures and physicians try to keep up to date due to their academic status.
I do also believe there has been some erosion in physical exam skills in modern physicians that come from less reliance on them. If you can get a chest film to diagnose pneumonia, why waste 10 minutes percussing on a patients back? In reality, the sensitivity and specificity of percussion of a skilled physician is probably significantly less than a chest film, and if you're not facile, then forget it.