Living one day at a time comes from the serenity prayer, but not drinking just for today (but perhaps tomorrow) is such a reprieve for the addiction that it instinctively latches on to that idea, grateful to still have a chance to drink every day, despite all the trouble it has led to.
Not drinking "just for today" is a daily reprieve -- for the addiction. This is precisely why people promote that idea. Heartcore's post about requiring lots of re-evaluations and having a giant bag of sobriety chips going back to 1986 is a disheartening example of this.