Worry not, for it is being built.
You will not catch me using the words "agentic IDE" to describe what I'm doing because its primary purpose isn't to be used by AI any more than the primary purpose of a car is to drive itself.
But yes, what I am doing is creating an IDE where the primary integration surface for humans, scripts, and AIs is not the 2D text buffer, but the embedded tree structure of the code. Zed almost gets there and it's maddening to me that they don't embrace it fully. I think once I show them what the stakes of the game are they have the engineering talent to catch up.
The main reason it hasn't been done is that we're still all basically writing code on paper. All of the most modern tools that people are using, they're still basically just digitizations of punchcard programming. If you dig down through all the layers of abstractions at the very bottom is line and column, that telltale hint of paper's two-dimensionality. And because line and column get baked into every integration surface, the limitations of IDEs are the limitations of paper. When you frame the task of programming as "write a huge amount of text out on paper" it's no wonder that people turn to LLMs to do it.
For the integration layer using the tree as the primary means you get to stop worrying about a valid tree layer blinking into and out of existence constantly, which is conceptually what happens when someone types code syntax in left to right. They put an opening brace in, then later a closing brace. In between a valid tree representation has ceased to exist.