The GPU they were licensing from Imagination was already very tailored to their needs. Apple already had a strong saying in the architecture, in the driver, in

The GPU they were licensing from Imagination was already very tailored to their needs. Apple already had a strong saying in the architecture, in the driver, in the compiler and in the implementation details of the GPU they were buying from them. Moving to a "custom GPU" was basically Apple saying "Ok, thanks, we are taking over from here".