> the kind of application it enables (near-native speed) can be had with ordinary native apps as well. Why does something like that have to run in the browser?

For the same reasons all the other kinds of web applications are not developed as native apps as well. Hint: it's not only about speed. Some of the main reasons web apps are winning over native apps is easy of distribution, updates and being multiplataform. And it's being increasingly harder to force javascript to behave as a general application programming environment.