I imagine IRC has the excuse of wanting the UI to be a simple line-based system, and thus in-band signalling is an unavoidable evil.

This same reasoning is why mail and other messages have the Header: Value format - you can compose in plain text. This is documented at least as far back as RFC 561, in 1973. Again, that's a good excuse: Users must compose by hand. Also, the RFC is just codifying what people were already doing.

HTTP, SIP, and others do not have these excuses. HTTP headers are essentially never written by hand, and the extremely few times they are, we don't need conveniences such as comments, line folding, lenient grammar, etc. (Proof: People write XML and JSON by hand far more often, without major ordeals.)