Formal proofs predate how we think about unit testing (check out IBM's 'Cleanroom' process). The concept comes from an age when most programming was a lot more

Formal proofs predate how we think about unit testing (check out IBM's 'Cleanroom' process). The concept comes from an age when most programming was a lot more like applied math, and it still has a lot of traction in industries with very high reliability/correctness requirements. Unit testing took over because it's easier and cheaper to do, the results are good enough for most environments.