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Because only MP4 and WEBM video formats include the audio when downloaded. And MP3 would be cool as this is the most standard audio format.
I'm sorry to disappoint you, but the script can only show video/audio streams that YouTube supports.
When you click on a link, you are downloading directly from YouTube.
YouTube only keeps MP4 and WEBM for legacy devices. It has never supported MP3.
If you want the best audio, it is best to download the Opus format. It has the highest bitrate (128 - 160 kbps) and 20 kHz frequency cutoff.