It's a dual-edged sword, for them. They're evidently trying to offer you lower airfares and also trying to avoid the situation where you're subject to schedule changes. Personally, I'd prefer to book my flight and deal with schedule changes, but I've read lots of messages from folks here on the DIS who are clearly very upset when airlines change schedules on them. So Southwest is appealing to those folks who are content to make their reservations later, but have assurance that the schedule will not change. Therefore, any system that would resolve the issue you're raising would cause dissatisfaction from schedule changes or substantially higher costs.