Airlines don't set their actual flight schedules until four or five months prior to flight time (some even less). That's where the timeframe comes from.
So your question is basically, "Why doesn't Southwest let customers book against forecasted flight scheduled?" They've never answered that question straight-out, as far as I recall.
There are many possible explanations. This way, they don't have to deal with the extra work involved in dealing with the case when their forecast was off, and therefore they would need to inform customers that their flight times have changed. They also avoid all the overhead involved in folks calling to check to see if flight schedules have changed, and calling to change flights because flight schedules have changed.
