Because you don’t hear the individual dots and dashes. After a while, you hear them combining into words. That’s how I learned it. It starts to sound like a monotone song.

Source: me. I used to be able to send and receive upwards of 20 words per minute. That was many years ago, and I’ve lost a good bit of it. But if I hear a snip go by in a movie or something, the words still jump out.