Right - an embedded webview with some window dressing. But it seems like they don't want to handle it, as they've made clear. So why not shell out a few bucks for one that's already made?
That's not an insult, it's supremely awesome that this is even possible and that this dev made it happen. I'm using it now. But there are a few of these types of things for Google Music, there's a similar Chromebook app.
Because they would need to support it. Plus all the other reasons why they don't have a native desktop client now. Like the strategic goal to establish Chrome as an app platform.
But this person might land a job at Google, like the Sparrow folks... http://sparrowmailapp.com/