Another reason is that tax havens (such as Cayman Islands, or within EU also Luxembourg or London) often have substantial amounts of financial services - lawyer

Another reason is that tax havens (such as Cayman Islands, or within EU also Luxembourg or London) often have substantial amounts of financial services - lawyers and other experts are available. So, for the logistics of running headquarter functions, it's not necessarily that taxes are low, it's also that there's available expertise for operation.