Most games are basically throwaway projects. You write it once, maybe put out a DLC, and then you move on to the next one. There's no incentive to keep the codebase maintainable long term, because there is no long term (unless you're making something like WoW). Games also aren't deterministic by nature, so some glitches and imprecisions won't even register as bugs to players.

I'd argue a database engine is far more complex than a game engine. It has to be 100% reliable, and the codebase needs to be clean enough to stay maintainable for decades.