It's a cold wallet. It's supposed to be the storage of pretty much everything, or at least a majority of the resources they control. If they have multiple cold wallets, that means they would employ multiple security measures, and focusing your attention to a single wallet is better than dividing your attention, resources, time and effort in guarding funds. Hackers would have a higher chance into cracking into multiple wallets, too, rather than a single, heavily-guarded one.