GT has done this for every version sine its 1st. This is most likely what inspired Turn 10 to do it.
I guess the concept is that when your racing your not as concerned with visual fidelity as you are watching your race...
Consoles have their limitations but other racers out there don't have to do this and still create a tremendous consistency and racing/driving sensation...
This is also what birthed the photo mode and show case garage modes, so they could show off the high res models that they used to make the lower res ones.
It just is what it is...
But here is the real burn, look at shift, looks pretty good, look closely at the cars, they are very low Polly with few rounded lines... they are normal mapped, something that takes awhile to do but still is more modern concept than model swapping.
For those that don't know what normal mapping is its basically taking the textures from a super high res model and mapping them to a lower res one. It's a LOT of work but the results are fantastic.