I dont think its due to intentionality just that low pitch is easier to cancel than high pitch things like voices.
But what if the software/ai is advanced enough to reproduce a sound, but erase a certain aspect of it? Like how photo editing can edit out an object or background? Then the headphones can use a seal to completely block out all noise, and play only the sounds the user selects!
Earplugs below my bose headset work wonder to block out anything short of cataclysmic, but it's hardly practical. And I can't exactly play music with those on...
The problem is you can’t possibly work out exactly how and when the sound will hit each set of ears around you to be able to direct a beam of sound to their ears (assuming it’s even possible to have such directional sound)
The problem is significantly easier with headphones. You have a headphone and speaker in between your ears and the sound. The distance between the microphone and your ears is constant so you can perfectly time the negative sounds.
Directional sound is possible to an extent, Woody Norris gave a TED talk about it a long while back, it is called hypersonic sound. I think it uses ultrasound to generate compressions and rarefactions far away. I realize it sounds like science fiction, but you could perhaps transduce sound to electricity and again to sound, sending the signal of your voice away from you faster than the speed of sound, in time for a cancellation wave to be generated and have an effect. I suppose the analogy would be quiescing a ripple in a pond.
The utility is limited by the limitations of passive noise attenuation devices. These things are great for the shooting range, because bringing gunfire down to a level that won't damage your hearing is pretty easy. There's no kind of earphones that can just completely block out all sound, though.
That is simply not true - at least in my experience with an old model of Sony headphones. It's possible that prediction plays some role, but I bought mine with specific purpose of silencing neighbor's kids.
They work really well with this kind of sporadic, random noise and the difference between turning on active cancellation and just passive attenuation due to ear muffs is very noticeable. I'm talking about low-frequency noises that are mostly transmitted through the vibrations in the walls and floors - feet hitting the floor while running, ball bouncing of the walls and so on.
Only few care for the latter and millions/billions care for cancellation of external sources.
Besides, "cancelling the human voice" would just provide a false sense of security, as said voice would still need to be received by others -- and like any message it could be intercepted.
Worth noting that the feeling of privacy is a lot about perception. People feel better if they know those they see around them cannot hear their private convo. That is why they speak softly and put their hands to their mouth talking in public places (the other part was to boost the signal, before the advent of differential microphones).
Don't get me wrong, I most definitely agree external cancellation is terrific, be it active or passive. In the passive realm people use traditional dampening materials, as well as MPPs (micro-perforated plates) in certain industrial applications.
If you want to cancel another human, just build a wall.