Comment by gjm11

It's much easier to arrange for your test code to be extremely modular, with different bits not affecting one another, than it is for the actual application code. And for most problems it's much easier to get evidence that you've solved them right than it is to solve them right. (It's a bit like the difference between P and NP.) Therefore, writing good test code is much much easier than writing good application code. This is the answer to your last question, and the reason why the rest of what you said is wrong.