I speculate that it's because it would be easier for humans to read and write boundaries than count bytes.
Multipart predates http/1.0 and was written for email. It wasn't unheard of in the early days to directly enter SMTP commands. It would also be more readable on clients that didn't support mime.
Multipart predates http/1.0 and was written for email. It wasn't unheard of in the early days to directly enter SMTP commands. It would also be more readable on clients that didn't support mime.
https://datatracker.ietf.org/doc/html/rfc1945#section-3.6.2
https://datatracker.ietf.org/doc/html/rfc1521#section-7.2.1