Game code specifically is usually the exception to this rule, because it tends to be written to very strict deadlines, ad-hoc requirements, short project lifeti

Game code specifically is usually the exception to this rule, because it tends to be written to very strict deadlines, ad-hoc requirements, short project lifetime (no need to maintain or support it for years after), and not much expectation of re-use. This does not all hold for Doom3 as it was also an engine sold to other companies for other games.