you don't write tests it means that you don't refactor your code because to refactor code you need a very good automated persuade and because you don't know factor code it means that your code starts to accumulate garbage and because your code starts to accumulate garbage it takes you a little more time to actually build new features and because it takes you more time to build features you get more time pressure and you add more time pressure so you have less time to write test closing a vicious circle cycle that always end up with the same