Most, if not all, police academies teach cadets how to disarm an attacker. But... there's no requirement to continue that training at all, so after those lessons are over they're forgotten. It's not like they're getting up every day practicing forms/katas or one-on-one sparring every day before their shift starts so it never becomes second nature to them and it never becomes something they can rely on when the situation arises.
And even if the situation arises, that person can still end up stabbed. When it happened to me, I had every single advantage over that nigga you could think of and I STILL got stabbed and sliced before disarming the nigga and folding him up.