Great question! And I'll endevour to provide an answer that explains our design logic here.
The main reason for that is that all those engine design choices have complex and interesting results for the final engine statistics, and have lots of interesting tradeoffs, improving certain aspects at the cost of others, and interacting with other design aspects of the engine.
We've come to the conclusion that most of the design choices that are just spending money to make one stat a bit better are not worth modeling in extreme detail.
For example ABS ECU generation would just be a bit more cost for a bit more safety, no real trade offs.
Number of speakers would just be a minor bonus to how luxurious the car is, for spending a bit more money and weight etc.
Most of the interior choices boil down to a balance of comfort, cost and weight.
Uncomfortable, cheap and light. (cheap budget car interior, dodgy plastic and cloth etc.)
Comfortable, mid priced and heavy. (Maybe a mid range Audi)
Very comfortable and luxurious, very expensive and heavy (think Rolls Royce interior)
Comfortable, Lightweight and Expensive (think supercar interiors)
Right now they don't have descriptions of what items are contained within each package, but they will.
Those are the only really interesting mechanics around interiors, and in our case we decided the best way to handle that without overly complicating that aspect of the game was to just give a range of interior packages. If you want to get ahead of the competition you can research ahead and use more modern interiors.
The gameplay mechanics in car design with the most detailed and interesting tradeoffs are choice of body (size, weight, comfort, handling, cargo space, aerodynamics etc.), drivetrain location and driven wheels, gearbox configuration, suspension type and suspension tuning. Those are the aspects we chose to focus on more heavily, as they're a really complex web of interesting choices with effects on multiple stats at the same time.
Pleb is quite correct, we did have more detailed interior choices, but breaking it down into more options just gave you a more fine grained version of "spend more money, get more luxury" and it wasn't any more fun than just choosing a broader option, and took longer for the player, as well as taking up lots of UI space
