I'm personally surprised by this too. Like, everyone is writing how insanely productive AI is making them, but that productivity doesn't seem to have translated into any innovations beyond model quality.

Like, most of the stuff needed to make AI better is stuff that could have been written by hand in 2015, so why hasn't anyone used their agents to do so?

To be fair, there is probably a way to make it work the way you want. You could add an MCP for a task queue and let the model work each item in the task queue. The tasks could be added by a deterministic system i.e. your harness.