As a testament to their (the original id software team that had both Johns) great efforts and legacy, they pretty much invented the concept of a game engine to start with. At that time, most games were written as a one and done type of a package deal.

Given that the team wanted to push the technical edge with the games consistently, they wanted to have some reusable/modifiable core that they could use across multiple projects, have visual level editors, etc., they converged on “accidentally” creating a game engine and coining that term. I forgot which game was the one that led to the creation of it for them, iirc it was one of the earlier Commander Keen games, but my memory might be failing me here. They didn’t even have the goal of creating a game engine, they just ended up getting there and then realizing what they did as they were trying to build their reusable “toolset.”