I too have noticed this. One tool that I find useful is https://getawayfinder.southwest.com/flights. You can choose a departure city and then keep the default "anywhere" as the destination...then pick a Tues or Wed (or a desired date) to see where you can fly for a cheap fare. For example, choosing "From SFO" and "To Anywhere" for most Tues and Wed in May 2015, you get $39 fares to Las Vegas. It is currently available only within the continental U.S. (not int'l).