A good part of that would be because GPS satellites orbit considerably higher than all LEO stuff, at approx 20,000 km, so everything in low earth orbit is almost equally as blanketed in valid GPS signal as things at 0 meters MSL on the ground on a beach.

Something orbiting at 550 km is approx 0.027 of the total orbital altitude of a 20,200 km orbit NAVSTAR series GPS satellite.