Parts = materials. Most car parts are made by automated process with very little labor costs. Here is a youtube on how engine blocks are made. Notice the only human interaction is the inspector at the very end of the process.
How it's Made: Engine Blocks
No labor costs to build engines.
THEN WHY ARE THEY SO DAMN EXPENSIVE! Steel and aluminum is what, $2/lb? - engine should be under $1000, but a new engine is MUCH MORE.