Speaking from personal experience and talking to other users - the agents/harnesses of the vendors are just better and they are customized for their own models.
Presumably the harness cant be doing THAT much differently right? Or rather what tasks are responsibilities of the harness could differentiate one harness from another harness
Examples that come to mind are how the context is filled up and how compaction works. Both Codex and Claude Code ship improvements regarding this specific to their own models and I’m not sure how this is reflected in tools like Cursor.