Not all usage scenarios are the public internet, and something doesn't have to be useful on the public internet to be standardized.
Realistically, systems for the public internet will use a secure hash as the cache key so it'll always be the same size. The cache key already includes a URL that can be very long, and an arbitrary set of header values.
Except that by definition, in a URL the data has no implicit meaning so for a cache hit you need an exact match, including order and case, but for a list of POST parameters, they could legitimately be in any order and so you can't just hash it all as a blob, you need to sort the keys, possibly copy data around (unless using keys plus hash), probably allocating more memory, etc. I'm pretty certain we'll see at least one CVE out of the first few implementations of this!
POST/QUERY data can be in any format. Who are you to say order doesn't matter? Are you sure you can even parse it? Mine is in DES-encrypted (with key "password") base85 DER, you really gonna implement that in your proxy?