That is exactly how it used to be. Mods _were_ part of the core gameplay. In fact they completely defined all the gameplay and the UI. See quake C, UnrealScr

That is exactly how it used to be. Mods _were_ part of the core gameplay. In fact they completely defined all the gameplay and the UI. See quake C, UnrealScript, along with a few others from that era. Counter strike was a mod. So was team fortress. That era is gone. Devs want more control now and most games have elaborate cash-shop business models. They don't want the gameplay to be modifiable.