Because Google Chrome contains non-free parts (internal PDF, Flash and media support)

As you may know, the Chromium browser is available in Ubuntu Software Center. Google Chrome is built from the same source code as Chromium, but includes some additional features which make it impossible to distribute via Software Center/the official repositories.

Specifically, it includes proprietary PDF/Flash plugins as well as support for AAC/MP3/etc. formats. These components are non-free (no source-code available), so Ubuntu's policy forbids distributing Chrome via the Software Center/repositories.

Chrome also requires agreement to a restrictive EULA (license)

Additionally, while Chromium/the source has a free license (BSD), Google requires you to agree to a much more restrictive "End User License Agreement" (EULA) to download and use Chrome.

Google forbids third-parties from distributing Chrome

I believe Google also prohibits anyone (or any site) from redistributing the full Chrome; that's why the "portable Chrome" packages you find will download Chrome from Google when you install them, instead of including it. Even Download.com only gives you the tiny installer which downloads Chrome from the Google servers.

Solution: download it, or use Google's PPA

The solution if you must have Chrome, as the other answers have explained, is to download Chrome or install it via Google's own PPA