The same reason people don't just write in assembly (or more accuratley, C) to write a desktop application very often. Javascript is the assembly language of the web.
If you can output JS that's fast enough, why not?
Java programmers don't want to learn JavaScript
It comes with good UI building tools
If GWT didn't have good UI building tools, Java programmers wouldn't have any excuse not to learn JS and program normal web applications. But here's a tool that lets them use their existing Java knowledge to build halfway decent web GUIs.This is just another example of how technologies are chosen by considerations other than objective technical merits.