My best guess would be that plugins are limited into what they can do within VSCode, and rewriting a whole IDE/Text editor just for AI Agent seems a lot of work.
For example a while back vscode-pets[1] plugin became popular and tried it and noticed that the pet can only live within a window, whether its the explorer section or in its own panel, I thought it'd be more of a desktop pet that could be anywhere within VSCode but apparently there are limitations (https://github.com/tonybaloney/vscode-pets/issues/4).
So my guess is that forking VSCode and customizing it that way is much easier to do things that you can't with a plugin while also not having to maintain an IDE/Text editor.
For example a while back vscode-pets[1] plugin became popular and tried it and noticed that the pet can only live within a window, whether its the explorer section or in its own panel, I thought it'd be more of a desktop pet that could be anywhere within VSCode but apparently there are limitations (https://github.com/tonybaloney/vscode-pets/issues/4).
So my guess is that forking VSCode and customizing it that way is much easier to do things that you can't with a plugin while also not having to maintain an IDE/Text editor.
[1] https://github.com/tonybaloney/vscode-pets