Not allowing a customer chassis is kind of the thing that makes F1, F1 though isn’t it?
Build a car and come racing.
If you can just rock up and buy last year’s Ferarri and go racing it would destroy the sport, there is no reason for even mid table teams to make their own car. So you end up with a small band of car manufacturers who dominate the sport and a load of also rans struggling by using their cast offs a la Moto GP. 3 car teams would also be inevitable.
If a customer chassis was an option 10 years ago it’s doubtful a team like Red Bull would exist as it does now