If a game update goes wrong, they can just send down another update. If a Steam update goes wrong...

(This has sort of happened. Valve managed to roll out a version of the launcher for Linux a while back which used instructions not available on older CPUs, and crashed before it reached the point where it could check for updates. Was a pain. Normally, if Steam exits uncleanly they force an update check immediately on next restart, but if it never gets that far you're screwed.)

Self-updating on Windows is a horrible thing to get right. I wouldn't be surprised if they did this to keep the complexity of the process as low as possible.