Just my feeling:) 1. It's better to run web application with more screen space, not every app need menu bar, toolbar, etc brought by every instance of browser. 2. For browsing, tab is excellent. For web application, tab is not enough. For example, I can't implement an "always on top" thing using tab, I can't let browser remember every application's location and size. These features are good for productivity.

Current browsers are designed for maximum surfing productivity and not dedicated to provide a perfect web application container.