No assembly lines for magic
It sounds like you've already solved the problem without realizing it.You said that magic requires individual enchanters building the object. You can also make it that the entire enchantment must be done by the same person or group of people - you cannot have one person create one part and another person create another part.
Let's say it takes a team of ten mages five years to create an engine. This means that, at a minimum, the total cost of the automobile engine must be more than the total wages a single magic-engineer earns in 50 years. (Plus all the additional expenses, like material costs). There will be no way for a typical wage-worker to ever afford a car of their own.
While the automobile was invented in the 1800s, it didn't reach widespread use until the 1900s, when the creation of the assembly line made their mass-production significantly more efficient. If the nature of magic precludes the use of assembly lines, there is no economy of scale - that is, the production cannot be made cheaper or more efficient by building additional equipment or hiring more workers. While cars may exist in your world, they will forever remain toys for the extremely wealthy and will never reach the widespread use they have today.