Fintechs can be agile
because if they screw up, it doesn't cause billions in loss, and barely anyone hears about it. They can push out change after change, fix on top of fix constantly.
If you are big bank, billions can be lost in no time due to the smallest error, and everyone will know. Stability, accuracy, and robustness, over quick change any time.
I'm reminded of a piece i worked on many years ago, an actuary came in with a hugely complex, recursive, formula for working out a critical (legally important) value. When testing the output, it was compared to an Excel spreadsheet someone in the test team knocked up in an afternoon, and the differences were flagged up as failures of the software. We had to then school the test team on how the whole point of doing this work, was that a spreadsheet couldn't work it out accurately. floating point approximation threw off the result enough to get us fined huge sums.
I don't think you could code it in anything but COBOL, with some serious computing power behind it, to make it work at all. Even C would probably have you delving into assembler to do it.