Formal methods are not used everywhere because for most real problems writing down what the function should do is itself a nightmare, and a program that is right ninety-nine per cent of the time is good enough.

when you start talking about like most interesting domain problems, you have to pull in so much context that basically even writing what the function is supposed to do becomes a nightmare. The imperative program you write that will get correct 99% of the time is probably good enough to use in almost all cases.