HTTP 1.1 is an old protocol, over time new requirements made modifications necessary, some things fell out of use, and some changes turned out to be mistakes. That it's text-based without using doesn't help

The basis is simple, but then add Cookies, HTTPS, Authentication, Redirecting, Host headers, caching, chunked encoding, WebDAV, CORS, etc etc. All justifiable but all adding complexity.

Http/0.9 is pretty simple, but for a fast web we needmore complexity.

More parsing and data processing = faster web. It all makes sense, really!

Joking aside, some "features" in HTTP/1.1 are really questionable. Trailing headers? 1xx responses? Comments in chunked encoding? The headers that proxies must cut out in addition to those specified in "Connection" header except the complete list of those is specified nowhere? The methods that prohibit the request/response to have a body but again, the full list is nowhere to be found?

All these features have justifications but the end result is a protocol with rather baroque syntax and semantics.

P.S. By the way, the HTTP/1.1 specs allow a GET request to have a body in chunked encoding — guess how many existing servers support that.

Nearly all L7 protocols and their parsers are complex. HTTP is kind of simple, relatively speaking.