It's to ensure that it's actually adhered to. Unfortunately experience have shown that it's unreasonable to expect all developers to remember this on their own

It's to ensure that it's actually adhered to. Unfortunately experience have shown that it's unreasonable to expect all developers to remember this on their own accord. I generally also advise the leads to make sure there is a reasonable limit to the length of a test method. As you say if it's too long it probably needs some work before it's allowed in the main branch.