This has to do with the nature of GTS, which is an easily updatable game, but above all the ease of development that this console has.
In the ps3 era (with everything working on a powerful console) they worked on too complicated hardware, which had to take too long and with many compromises to develop certain things. In that generation the PD has suffered a lot in terms of development.
With ps4 they have had a powerful machine that is simple to work with and starting from this solid base, they have created a new graphics engine (I think they trashed the one used on ps3 or otherwise heavily modified it) making it easily updatable and easy to modify.
For this I expect that on ps5 will have much less work to do for a possible GT7, because they will only need to update, where needed, what they already have (texure, polygons on the track, more cars, new particle effects, dynamic skybox like on driverclub that project dynamic shadows, best AI, etc.).
On ps5 I expect something crazy visually.