Electron.
@gruber @stroughtonsmith They also end up in compressed memory (essentially paged out) or evicted entirely from memory with only a screenshot of the screen remaining (so you see something in the app switcher).
Most apps are written to take that into account, but on macOS, the model is different and an app's state is much more visible than the app switcher: it would be weird if the News app just randomly disappeared from the Dock, for example.