I don't believe its entirely a money problem. Certainly there is a factor but that oncologist is also spending a few more years in Residency typically. The trade-off is that as a PCP you are working predictable hours in generally a low stress environment.

So I agree money is a factor but I also think the current insurance model makes its incredible difficult to run a PC office. You have to bring on significant overhead which you then need to cover my making strict quotas on seeing patients. I don't believe this aligns with what PCP enjoy doing.