It's also an usability nightmare, as a lot of things that Steam is used for both update very often (weekly to daily), and always need to be kept updated to the newest version or they fail to start. You'd have to run a apt-get update && apt-get upgrade every time you launch your favourite game.

Most likely, Valve will provide a steam_installer.deb, which depends on all platform libraries Steam needs, and which only contains a script that creates the necessary users and rights, reserves some directory for steam, fetches the actual Steam off a Valve site, authenticates it, and runs it.