a) Noise reduction usually requires a powered component, or some kind of neck brace to contain the battery required for active noise cancelling. This adds expense and weight.
b) Sometimes it's good to be aware of your surroundings! When I'm listening to music, noise is usually sufficiently blocked by the sheer volume of the track, and during podcasts and audiobooks I'm also not bothered by the sounds of the streets.
If anything, it's good to hear a siren, or the ramblings of a nearby crazy person in order to avoid them.
The only time I find noise cancellation useful is on airplanes.
b) Sometimes it's good to be aware of your surroundings! When I'm listening to music, noise is usually sufficiently blocked by the sheer volume of the track, and during podcasts and audiobooks I'm also not bothered by the sounds of the streets.
If anything, it's good to hear a siren, or the ramblings of a nearby crazy person in order to avoid them.
The only time I find noise cancellation useful is on airplanes.