This is a good point, but just a standard for the standard create/read/update (replace)/delete operations combined with some baseline guarantees (like approximately-last-write-wins eventual consistency) would probably cover a whole lot of applications that currently use S3 (which doesn't support appends anyway).
Heck, HTTP already provides verbs that would cover this, it would just require a vendor to carve out a subset of HTTP that a standard-compliant server would support, plus standardize an auth/signing mechanism.
For example, "Can one append to an existing blob/resume an upload?" leads to lots of questions about data immutability, cacheability of blobs, etc.
"What happens if two things are uploaded with the same name at the same time" leads into data models, mastership/eventual consistency, etc.
Basically, these 'little' differences are in fact huge differences on the inside, and fixing them probably involves a total redesign.