I guess it depends on how you want to term it.
Games like Rust use Unity as the engine behind their game.
Games like Eve Online, which is created primarily in Python, the game is the engine.
To coders, it's semantics...
To the public though, it's public perceptions and thus, marketing. When the word engine appears, people think of that thing under the hood of their car that makes it go. To most, it's a mysterious, mechanical marvel that they can stare at in wonder and admire, but don't have to think too much about. Calling the code behind a game an 'engine' imparts that same mystery.
It's the same exact reason that the food industry wants to change 'high-fructose corn syrup' into 'corn sugar': public perceptions and thus, marketing.