Clearly you haven't looked at my CV. I spend about half my day reading this country or that one's regulations to make sure that the small bit of the worlds largest CSD (by at least one measure) that I work on follows the law, and I have the insatiable curiosity to read the rest of it as light bedtime reading. I know whereof I speak regarding financial systems. You don't.

Specifically, accounting law is old. Computers were used to automate what the bookkeepers did by hand, which was emphatically not floating point. At the end of an interest period, interest was computed and booked in rounded form. The next period, interest would be computed based on the number in the books, not the "floating point" number that was rounded from the last time interest was booked. More clearly: put 1c into a bank account earning 1% interest compounded daily. After 1000 years, that account will have precisely 1c in it, because there was never enough interest to earn at least one quantum. This is the difference between math and bookkeeping, and between the sort of math that passes for correct in an ERP system versus the sort of math needed by a bank.

Also, fixed point is not slow, at least not on the systems relevant to actual financial infrastructure. It's all on IBM Z, which has hardware instructions for fixed point decimal calculation because if it didn't, it would run like treacle.