To counter myself, I could also easily imagine that game engines are also full of dirty hacks to get things that the front end people want done, now, by hook or by crook. Ie, if a nice sane interface doesn't exist, and woukd take more than 11 minutes to add the right way, then just butcher in any kind of tight coupling and to hell with worrying about it breaking for the next game.
Oh no, I think you made a good point. I was definitely thinking about game code per se, rather than engines. The extra rigour around an engine (any library code, really) is definitely a good thing