Aston dont do everything inhouse so that limits Newey and can cause frustration for his creativity. They depend on Mercedes for engine and gearbox.
If Newey wants pull-twist slinky rod suspension on the rear, he wont be able to get it on the car if Aston doesnt make their own gearbox or engine.
There are too many supplier and business structure limitations with Aston. ( Unless they are building their own geabox in 2026)
Whereas with Ferrari, they make everything themselves and have full freedom to influence the whole car.