because the tooling for VS code extensions to be part of the VS code UI is lackluster. It shoehorns people and prevents the development of something like a fully integrated SQL workbench. Or a form editor. Or reusable property panes, editor widget UI. If vscode had those things, it would be a proper IDE.
Take for example this outline view: https://raw.githubusercontent.com/eclipse-theia/theia/master... - not something that would be easy to integrate into VS code as an extension. Very nice work. Why VS code doesn't have it is probably related to cannibalizing other MS products.
Odd example - VS Code already has an outline view, and it’d be very easy to build one yourself as an extension if you wanted. (TreeViewProvider API).
With the addition of Webview Editors and Views there’s not really anything an extension can’t do to its UI. Notice this is very different from saying there’s not much an extension can’t do to VS Code’s UI. Extensions are given a box, and they gotta stay in it. Personally, I’m fine with that.
