Because they originate from a time where plain text was what you had, and the format was created to be consistent and easy to format by hand. I still prefer my specifications to be in plain text so that I can view them easily and quickly.
I mean, they are formatted in HTML now, but as a giant block of monospace text. They could use <hx>s for the headers, let the text flow with browser resize for small screens, heck use bootstrap for good readable web typesetting, etc.
I am not sure why the IETF isn't interested in better web typesetting for em.
At one point someone had a site that did that, I think it was advertised and discussed here on HN, but I forget what it was called, and I think it faded into non-supported obscurity (since it was obviously a labor of love, unpaid).