Prior to ~2010 we were simply deleting source code and assets for finished projects; either because they weren't owned by the developer due to a publishing deal, or because the developers didn't want to reuse their garbage code. Same follows for assets, often they were owned by the publisher and not the developer, but if the developer did happen to own them they'd rarely see reuse in future projects. And publishers didn't catch on to the value of data retention until remakes started to make serious money.