[danbolt]

I'm a little surprised you're being downvoted. There might be a bit of hyperbole in your words, but no game engine is entirely off-the-shelf per-game. Or, idiomatic code suited to an engine always requires a bit of elbow grease to be ready for shipping.

[djmips]

I'm being downvoted, likely, by people with little experience in this particular area I guess... There is a real actual gulf between a fully tuned AAA game engine and what you can buy.