not all RFCs are standards. RFCs that become one are labeled as STD and a number, but nobody uses those (afaik)

https://www.ietf.org/rfc/std-index.txt

https://en.m.wikipedia.org/wiki/Internet_Standard

Because the internet used to be unimaginably small. It really was just a couple of researchers talking to each other. They weren't co-located (the entire point of internetworking) so they had to mail stuff to each other, but it was more like when you send a copy of a document to your co-worker down the hall. "Hey, Jane, watcha think about this?" The first telnet (predecessor to SSH) RFC was 9 pages, including an example of the connection handshake.

Years ago when the internet was still a bunch of universities, the main few folks working in the network were often times tuning hardware like routers. I do not remember what the first rfc was about, but it was genuinely a "request for comments" document mailed it to others. Ever since then it stuck.

If you are interested history behind RFCs and early Internet, I suggest reading "Where wizards stay up late: the origins of the Internet" from Katie Hafner and Matthew Lyon. Thats good book about arpanet formation and how arpanet and other networks together became Internet.