I only live ten minutes from the border but I rarely go down, it's just not worth burning an hour or more of my weekend to maybe save $20 on a tank of fuel. Maybe if I had a big truck with a huge fuel tank it would be more worth it. Additionally my in-laws live outside of the metro area here that has additional fuel taxes, and they're only 30 minutes away, so when the wife and I go there ( a couple times a month) I can save on some fuel out there.
I tend to agree a lot of people just aren't good at math also, or people don't put a value on their time. Even a family Costco run that might save you $100-150 if you spend say $6-800 on a trip, is that really worth burning 1/4 of your weekend? I don't think so.