Safety implications are one thing, but for any evolutionary trait for us humans, we look back to prehistoric times for reasons on why they evolved into us. Just imagine someone of a small family or tribe having to get some form of food. We would need to be focused and attentive on the prey we were tracking, or the berries and herbs we were plucking, so it would be very detrimental to survival if we stopped such essential tasks to watch the leaves fall off the trees every five minutes.
That being said, it is also because of survival that our hearing would actually take on the role of paying attention environmentally during such times-the sight of a pretty leaf might not be important enough to detract us from bringing down that creature we're after, but the sound of a loud roar or thundering charge of a mammoth definitely would be.
In a scenario of inattention blindness, it wouldn't be that we wouldn't hear something, not if our ears were free. It is that we choose not to respond.
