The JavaScript is probably not inlined in their unbuilt source. That would be potential bad practice. By inlining it into the page source when built, the page won't have to make any additional HTTP requests to download a separate resource file, as HTTP requests are one of the slowest parts of loading a web page.
The sheer amount of JS is, by my guess, probably due to a lot of common libraries and code shared among many of their pages, and trying to have it not load on their homepage would end up involving a lot of unnecessary and nasty workarounds in their code. Plus, the UI may look simple, but how much is actually happening under the hood? There are Google accounts to worry about, instant and voice searching to do, analytics tracking, etc.
In the end, however, you probably won't truly know unless you get a Google engineer to divulge their secrets, or get a job with them.
