The software industry can absolutely achieve multithreading, and your CPU is definitely not designed to "work one core per time".
However, there's a difference between
multithreaded (can take advantage of more than one core) and
scalably multithreaded (can take advantage of as many cores as you can throw at it) application code, much of which revolves around finding an efficient way to do things like:
- coordinate access to large, complex shared data models
- minimize the amount of time that one part of the application spends waiting for results from another part of the application
- etc.
Stellaris is multithreaded (it will, clock for clock, generally run slower on a one-core system than a four-core system), but not scalably multithreaded (sixteen cores won't gain you much compared to four cores).