The difference is scarcity and readily available tools to retarget the steam engine for a different workload. A steam engine requires tools made of matter to make it provide mechanical power to another system. In essence, a steam engine is just a power supply.
A general purpose computer, on the other hand, includes a power supply, and generally doesnt need tools that change matter to retarget it for another application; the tools needed to do so are made of information, and are thus readily available.
Granted: we as CS folks and business folks are choking off our own sources of talent by hiding the tools and keys needed to truly examine our systems, all in the name of "user-friendlines", but its still possible to use what is exposed to learn computing basics like how wifi works, or what a proxy server are.
To be frank, I think this distinction is precisely why I get frustrated at computing incompetence: a PC at home isn't locked down and has access to these tools. Anyone can learn -- even using a web browser and notepad to write JS.
In contrast, learning how an engine works requires mass-based tools that are big and expensive and require careful knowledge of how to not harm yourself when disassembling or working on the engine.
This distinction is massive, and yet we still use analogies to cars. Shops with tools are not plentiful and readily accessible to average people, the engines can't be examined from the inside out, coils and springs are dangerous physically. I can't just go looking to take a class, either: not all schools have shops!