They could do a lot of new things while still making another Gran Turismo game. Its by choice at the top of the company they've been making primarily the exact same game for so long.
I can't imagine it was particularly motivating for game designers and ideas people to walk out meetings after each game having been told to program the same game structure, the same events, the same UI elements over and over again.
Sure, for the 3d modellers their scope is limited, and some other areas are always going to be iterative, but everyone else, blame the people at the top.