Distributed patching becomes hell. You need transactional semantics and files are not laid out well to help you define invariants that should reject the transaction.
There is no reason why the descriptor of objects can’t be updated with a new value that has all of the old chunks and a new one, since S3 doesn’t do deduplication anyway the other chunks may be resized internally with an asynchronous process that gets rid of the excess data corresponding to the now overridden chunk.