It’s because active noise cancelling is bad at cancelling sudden sounds, so many types of noise people want to protect against (gunshots, metal clanging at a construction site) would be poorly attenuated by current active noise cancellation technology. This is a not really a physics issue, just an active noise cancellation technology issue. Fundamentally active noise cancellation can and does reduce sound pressure, because the speaker basically “pushes against” the incoming pressure waves to flatten them out.