Because that's a totally reasonable thing to do.
It's not like "Hey I would like to display a plain-text version of this angry tag salad" is why multipart/mixed exists in the first place or anything.
My guesses on the thought process here, from least to most likely:
- Eh, everybody just generates their text/plain part by stripping tags anyway, without even the courtesy-reacharound of turning <BR> into \n so why bother?
- I tried that but then 90% of my mail notifications turned into, "Please use an HTML capable mail reader to view this message" because nobody generates text/plain parts but for unknown reasons they feel compelled to include them anyway.
- By the time we are putting that dialog on the screen, we are thirty levels above the last framework that had access to the MIME parts, because that's how software works now, so fuck it, fuck it all, burn it all down.
- Even though I'm on the team that writes a mail reader, I have not read RFC2046.
- Herp derp I eat paste.