They have these really, really long lists what all needs to be secured and how. Some of it is reasonable, some of it is bonkers, there is way too much of that stuff, and it overall increases the price of any solution 10x at least.
But OTOH I can hardly blame them, failures can be catastrophic there, as they deal with real money directly and can be held liable for failures. So they don't really care about security, and more about covering their asses.