I always thought it was because noise cancelling can only block repeated waves (like the sound of an engine whirring) but conversation is constant changing sound so it can’t be blocked.

Much easier to match pace with a wave with long wavelength than one with short

Yes, but the question was essentially, "does the sound it cancels out and you don't hear still hurt you". If you still hear it, you're answering something else.

High frequencies are passively attenuated by the foam inside the earcups. It's the mid frequencies (speech, etc.) that survive both active and passive noise cancellation. Still, a high end pair of ANC headphones does a good job on speech/tv if you have some soft music playing through them.