Most of your car's components are not monolithic parts. Instead, they're composed of many smaller and often more sophisticated elements. For example, your car's starter is a powerful electric motor with many potential failure points. In most cases, these parts are cheap enough that an internal failure means replacing the whole unit is more cost-effective than the labor involved in repairing it.
However, your engine is one of a handful of parts (along with your transmission) where this may not be true.