If a bank app wrote their own security code, and didn't use the standard platform library, then when a bug was found, they'd get piled on here for rolling their

If a bank app wrote their own security code, and didn't use the standard platform library, then when a bug was found, they'd get piled on here for rolling their own and not going with the much more widely tested platform implementation. Should a well-put-together banking application re-implement the entire OS? After all, there are lots of places where security bugs can hide throughout the OS.