Having tons of gold have got to be a nightmare. Moving it is a vulnerability. You have to guard it, and guards the guards.
Gold bars are assayed and authenticated and go in a vault, ordinarily they don’t move much, if ever, for the reasons described. The paperwork only changes hands. Once it physically leaves a “registered facility” or whatever, the chain of custody is unknown, it causes problems because it is no longer is guaranteed in a sense, the buyer will want another assay. Big bars have been known to counterfeiting. Since assay and casting into .999 bars costs money, the price paid is discounted.
Same thing with melting a bar down or scratching off serial numbers. Now what? If someone goes to sell an unrecognizeable blob they melted down in a furnace they will take a hit on the price paid. Sure, “it’s gold” but it will need to be sent off for assay and casting into another coin or bar, and eventually back in a registered vault somewhere.
The US origin bars are “coin melt”, literally melted down from US coinage and are 90% pure alloyed with copper. Pure gold is too soft to be used as coinage.