"Quality" of the harness matters a lot to the user experience, and the construction of the harness will depend on the behavior/quirks of the underlying model. So, if you're using Claude Code, you can expect it to work best with Anthropic models, and expect other model-makers to want you to use the harness they've developed.

But mostly vendor lock-in, I imagine.

There are different grades of vendor lock-in. There's mechanical lock-in (which is a thing, like .claude folders) and economic lock-in but then we don't pay enough attention to behavioral lock-in. Habit is powerful, and if you can habituate users into a certain flow, change feels bad and they are more likely to stay.

Why not? They are relatively easy to make so why not. Even I made one: https://github.com/computerex/z

implementing their own version of steganographic monitoring lol