The argument can be made that:
1. GT1-6 already had the classic campaigns you're looking for. Why continue offering the same thing?
2. There are other racing games on the market that offer the type of campaign in question. If the need for the classic rags-to-riches campaign is satisfied by the competition, why should PD invest more money into that?
3. How do you get the genre to move forward? It seems like PD and Sony don't want GT become the next Madden/FIFA/NBA with just roster changes and incremental improvements. Offering the same product as before means people will continue to ignore the advances and innovations they are bringing to their game, particularly with respect to community, online, and overall racing.
1: Just no. 4 years and a new generation of hardware brings advancements in just about everything to improve on the old. Better simulation, better track scans, new cars, better AI, sound, plenty to make the series feel fresh.
2: There is not. Forza seems close but once you start playing, it really pales in comparison. So much so that I think I'm already done with it. GT literally invented that style of automotive campaign and few have ever approached it.
3: I think you got this backwards, now we will just be spoon fed incremental updates as DLC with little major advancements more in line with annual games. The development cycle for GT has never been close to yearly. Each new game brought new things. A crafted campaign would not hurt any of Sports new features.