One of the reasons it isn't done is the QC problem. If you have a 10% rejection rate on 20 parts, 18 parts will pass. If they were all one mega-part, the whole

One of the reasons it isn't done is the QC problem. If you have a 10% rejection rate on 20 parts, 18 parts will pass. If they were all one mega-part, the whole casting has to be rejected. You then add in repairability issues and no flexibility to accommodate other types of manufacturing variability (errors, fit) and your savings from a single casting can become a big deficit instead. Sadly the reason this bad casting was shipped to a customer is probably because they are not achieving the desired pass rate so they knowingly wave through defective castings.