Isn't it obvious? To have a compelling product when GT8 hits the market. GT7 is a complete product, PD don't need to do the anything beyond a bare minimum for the remaining lifecycle of the title. It's compelling enough to continue selling to PS5 owners in the market for a driving title. I know some have this bizarre idea GT8 will be 5, 6, 7 years in the making - but that's clearly not going to be the case if you understand PD's workflow and carry over content.
PD are producing 3x-4x more content per month than they're releasing. By now they will have a good 50-100 cars in their locker and at least 3-4 new tracks. They're certainly not going to give all that away to existing GT7 owners who paid their £70/$70 2 years ago. That's locked away for a new £70/$70 GT8 purchase down the road - quite possibly with paid DLC to keep the revenue flowing.
Pretty much every notable road and race car from the last 3 years aside from the Toyota GR010 and Porsche 911 GT3 RS are absent from the game. That isn't coincidence, it's driven by market forces to maximise GT8's sales. Titles that are GAAS or only recently released are getting all these cars. PD and Sony are cynically limiting the scale and scope of their DLC. I don't blame them, but it's time we wised upto this fact.