Because it's useful to be able to access the same state from everywhere, and to not have to worry if the computer in front of you gets destroyed or stolen. Plus Chromebooks are more resistant to viruses and tampering.

Thinking about a Chromebook as a restricted computer is kind of like thinking of a USB keyboard as a restricted computer. Yeah, you can't really use it as a general-purpose computer. But that's not the point. It's an ultra-reliable toaster that you use to access some other general-purpose computer in the cloud.