Usually it is because people have a short term view of software, but in practice software lives for much longer than most people can imagine at the time of writing and that initial burst of productivity when working on a new system is even more present in a type-free and test-free environment.

But once that honeymoon phase is over you usually realize that types and tests were invented for a reason and beyond a certain level of complexity they are absolute must-haves if you want to have high confidence it all works as advertised and to be able to effectively refactor your code.