Comment by mauvehaus

The complexity in the engine isn't in the mechanical concepts that make it tick, it's in the implementation details. Stepping back quite some time in technology, but staying with engines:

The platonic carburetor is a dead simple device: a Venturi, a jet, and a butterfly valve. Real life carburetors are fiendishly complex: multiple jets, an accelerator pump, a choke. And god help you if you have multiple carbs on a single engine and need to sync them.

Everything that goes into making an engine work is similar: cooling it correctly and evenly, allowing for operation while parts expand and contract at different rates as the engine reaches operating temperature, lubricating everything, preventing vibrations that'll make the car feel unrefined or maybe tear the engine apart, valve timing (fixed in most engines at some compromise between performance and drivability), ignition timing (variable in most engines), sealing things that need to be sealed across a huge range of operating temperatures and in the presence of differing rates of thermal expansion (head gaskets, among others) oh, and making it work for a quarter million miles or more with fairly minimal maintenance. And manufacturing them at enormous scale, and holding the tolerances that make all of the above possible across the lifespan of the production line.

And all of that is before we even discuss pollution controls.