most GS-15s I knew grewing up in the DC area didn't have PHDs (and most PHD government employees I knew weren't GS-15s as GS-15s in research fields are rare)

As an aside (apologize for the semi humble-brag), I get a lot of mileage out of the fact that I'm the only person who was a GS-1 than many people have ever met, considering if one has a HS diploma, one is automatically a GS-2 :)

No. Just experience. Most 18Fs I know are GS-14/GS-15.