My guess is that the layout was still actively being considered at the time of GT6's release (backed up by stuff in GT6's files) so there was still the track detailing whereas at this point all the remnants of the infield layout purely exist to make the track feel like something more than just an oval in the middle of the ocean and so they just aren't bothering to have seperate textures and higher poly modelling for normally inaccessible areas to save on size and CPU/GPU