Why update today what you’re free to update in three months?
Here’s Valve’s reasoning.
Ah, yes, I can see that being a problem.

This is what that menu looks like.
This will let you apply it to all your installed games.
For me, downloading updates on launch would be a horrible existence.
By the time the updates are done, it’s bedtime.
There is no way in hell I want to turn Steam into that.
But, yes, I can see it being useful for some concerned with bandwidth.
Here’s predicting the feature will quietly and confidently make its way to the stable branch soon.