I've built many useful tools as single HTML files. And several of my larger projects have started as single HTML files and stayed that way for quite a while. Even when they get bigger they don't grow all the cruft that a "Hello World" Android app has right out of the new project wizard.
I think the issue is just about app scale. Android development is designed around medium or large apps, and doesn't at all try to optimize for small apps, and certainly not a Hello World app.
And honestly, I'd prefer that an IDE generate a Hello World app with all the random stuff in it that a full-blown app would need, even if it isn't required for Hello World. It makes the learning experience a lot more useful.
Single HTML files are a nice unit of component for building UIs. See the BrickLink store-front. Each part is defined in its own HTML file and then these are concatenated before being served. At dev time each component can be put through its paces in isolation ("explore the characteristic state space"). At runtime you can `clone()` the DOM to make more, modify it and so on. It worked really well.
