Developers Who Ignore Requirements

Just got off a frustrating call with a developer who ignored a very straightforward requirement and I only found months after the release. I've had this situation before, where I find a requirement that wasn't met and the developer tells me that, yeh, they decided not to follow that requirement. I understand that sometimes once you get into things that a requirement can't be met, but how do people think it doesn't need to be a discussion? I am not a developer or engineer, so am struggling to understand where this come from.