Because a lot of the value and development cost of an agent comes from the harness. So it makes sense to keep the harness code private, at least for now.
If/when everything stabilizes, we will probably see more open source. Agents are still very new and people are still figuring out the best way to make a harness
This is not good for open-source. Claude is not open-source, copilot-cli is not and antigravity-cli isn't either.
Apparently the major players decide to keep the secret agent source, well, secret.