Buying a vehicle piece by piece would cost several times the price of the finished car. The gap comes from how spare parts are produced, stored and distributed rather than from what they are made of.
Volume disappears after the production run
A component made for the assembly line is produced in enormous quantities, and the tooling cost is spread across every unit made.
The same part made years later as a spare is produced in small batches, sometimes by restarting a line for a single run, and the setup cost is spread across far fewer pieces.
That change in volume alone accounts for a large share of the difference between what a manufacturer pays on the line and what a workshop pays at the counter.
Availability has to be guaranteed for decades
Manufacturers commit to supplying parts long after a model has left production, and in many markets that obligation is set by regulation.
Meeting it means either holding physical stock for years or retaining the tooling and the supplier relationships needed to make more on demand.
Warehousing thousands of slow-moving items, most of which will never be requested, is a real cost that has to be recovered from the parts that do sell.
The chain has several layers
A part typically passes from the original supplier to the vehicle manufacturer's parts operation, then to a national distributor, then to a dealer or factor before reaching the customer.
Each layer holds stock, provides warranty cover and takes a margin for doing so, and the final price reflects the whole chain rather than the manufacturing cost.
Independent suppliers making equivalent parts sell through shorter chains, which is much of why the same component carries very different prices depending on where it is bought.
Modules replaced assemblies of small parts
Vehicles are increasingly built from modules that combine several functions, and a fault in one element often means replacing the entire unit.
That reduces assembly time when the vehicle is built and simplifies diagnosis in the workshop, but it raises the cost of any individual repair considerably.
Electronic modules add a further step, since many must be coded to the specific vehicle before they will operate, which requires equipment and access to manufacturer systems.
Demand is impossible to forecast precisely
Nobody knows which parts a fleet will need in a given year, because failures depend on use, climate and accident patterns that vary unpredictably.
Distributors therefore stock defensively, and the cost of holding items that never move is carried by the ones that do.
The alternative, ordering only on demand, shifts the burden to the customer as a long wait, which is why availability and price tend to trade against each other.