I imagine with the amount of money they manage they'd need to keep money in various countries in order to effectively do business in certain parts of the world. It probably makes little sense for a company to keep all their banking in a single country.
As a very simple example, how would a Canadian company open an account in GBP or even Euros at a bank in Canada? They offer US and Canadian denominated accounts only. So to accept these currencies they'd have to convert every time, lose 3%, then convert back and lose another 3%? They couldn't do that and stay in business.
So I'd say it's likely impossible for them to do all of their banking here.
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