But it does. Even for daily use, you probably need only a fraction of the interface - but even if you need it all, the tool is usually only part of your work.
You need other programs to do other parts of the same work - design in Figma, code in VS Code, collaboration in Google/Microsoft office suite, shitposting (er, well-being and psychological hygiene at work) in Slack, etc. AI sitting on the outside is able to operate all of them, combining their capabilities for you, to do what you want and how you want it. AI sitting inside a single product is limited only to the surface of that product, and is limited to capabilities the vendor wants.
You need other programs to do other parts of the same work - design in Figma, code in VS Code, collaboration in Google/Microsoft office suite, shitposting (er, well-being and psychological hygiene at work) in Slack, etc. AI sitting on the outside is able to operate all of them, combining their capabilities for you, to do what you want and how you want it. AI sitting inside a single product is limited only to the surface of that product, and is limited to capabilities the vendor wants.