Google started pushing software through Google Updater. Google Pack's help center gives some reasons why it should be convenient to use the Updater, but most of them help Google promote other software. "The Google Updater makes the software installation process more convenient in several ways. First, it installs software easily with just a few clicks. Also, once the Google Updater is installed, you can choose to have a system tray icon notify you when new software becomes available. Finally, the Google Updater provides you with a central place from which you can download more Google software, as well as other software we think you'll enjoy." (my emphasis)
This practice is not Windows-only. Google's Mac software is installed only with the updater. "Google Updater is the installer for Google products on the Mac. You can use Google Updater to see which Google software you have installed and to see other Google applications you might be interested in. Google Updater helps keep your software up-to-date by installing updates when they become available. And you can use Google Updater to uninstall Google Software." Probably the most outrageous part from the Mac FAQ is the answer to the question: how do I uninstall Google Updater? "To uninstall Google Updater, you first have to uninstall other Google software on your computer. You can't uninstall Google Updater while you have Google software on your computer because we need it there to keep your software up-to-date."
Google still offers the chance to install applications without the updater, but the page that points to the direct links is too difficult to find and has an inappropriate title. I'll repost the links here, for convenience.
Google Desktop for Windows:
Google Earth for Windows:
Google Toolbar 4 for IE:
Source:→ Google System
Google, Google Updater, Google PAck, Google Earth