In small series there is some room for correction.
In mass production there is no margin for error.
It has to be OK right away. During assembly there is neither the time nor the possibility to adjust even minor mishaps. Whether it’s cars, toasters or bicycle locks. Any small detail can cause major mayhem.
All parts are produced separately in large batches, each with their own production tolerances. So it’s essential that any possible combination of smaller/larger parts will always amount to properly fitting and functioning products.
A failure in just 1% of all products is a major problem.