There was a brief period of time where you could buy your car like this. You'd purchase a rolling chassis from one manufacturer, and commission a coachbuilder to put a body on top. Many premium brands such as Bugatti, Rolls-Royce and Jaguar (Swallow) started in this fashion.
Today, outside of a few niche areas such as motorsport and commercial uses such as buses and coaches, nobody buys a vehicle this way. If you walked into your local Ford or Toyota and asked for a rolling chassis they would look at you as if you were insane, and rightly so. Integrating the development of the chassis and body into a single unit (both philosophically and literally [0]) has given us cars which are lighter, faster, more efficient, more featureful and safer by every measure.
We had our coachbuilding period in personal computing and it's all but over[1]. Nobody asks for the hardware and operating system to be sold separately for their washing machine, their TV, their microwave oven, PlayStation or Tesla EV. And yet for some reason some still cling to the idea that tablets and smartphones are personal computers rather than recognising them for the appliances they are.
As Steve Jobs allegedly said, design is not how something looks, design is how something works. How a feature works on a highly evolved device like an iPhone is a function of tightly coupled and carefully designed hardware and software.
Having this design process take place in different teams inside different companies, selling in different commercial models would not lead to a better outcome, it would be worse, much worse. The staggering commercial success of both iPhone and iPad is all the proof you need.
If hobbyists want to hobby, more power to them! But it's not something any government needs to regulate into existence.
[0] https://en.wikipedia.org/wiki/Vehicle_frame#Unibody
[1] Servers/Linux are the commercial vehicles in this analogy