That’s a reasonable general rule but it’s too easy to remember the short version “make games, not engines” and forget the underlying advice, which is more subtl

That’s a reasonable general rule but it’s too easy to remember the short version “make games, not engines” and forget the underlying advice, which is more subtle. Making a game engine first and then making a game second is a fools errand, even if your goal is to make the game engine. However, you can make the engine in parallel with the game. Making a purpose-built engine in parallel with a game can be very reasonable. Having made a number of games with and without off-the-shelf engines, whether or not a custom engine makes sense depends heavily on the specifics of your team and the game you are making.