Before the online multiplayer aspect took over, we had a single player career in the form of Simulation Mode though. If not proper for everyone's standards, it's was the closest thing to one.
There used to be various events & leagues for players to choose from. We had leagues based on difficulty (beginner-extreme), we had rally events, we had the endurance events, regional events, manufacturer's events, missions, etc...
That's no longer part of the series, sadly. Sure, they brought in some events via the Power Pack, but that's not something that should have to be added in through paid DLC, especially not when it was once the bread & butter of the series. It's something that should be part of the overall experience from day one, period. Without it, it feels like Gran Turismo has lost part of its identity, you know.
I'll repeat the same sentiments I echoed in another thread, I have no issue with improvements made on the multiplayer side of things, but it's not right when the eggs get thrown into that basket while another equally important element which helped made the series what it is today gets neglected as a consequence. There's no reason both aspects of gameplay cannot co-exist. There will always be a playerbase who are more old-school like myself, who prefer the traditional Gran Turismo Simulation Mode, just like there are people who love playing online multiplayer. You can absolutely have both, and the series would be in a better spot for it imo.