Folks did this plenty prior to Electron, either by using cross-platform GUI toolkits (GTK and Qt both run on Windows, Java's been doing this forever with Swing and JavaFX, etc), or by writing GUIs for multiple toolkits/OSes that work with the same/similar core application logic.

Electron makes it easier to build cross-platform apps, and certainly cheaper, but it's not like it's the only way to do it.