Hubby flies all the time. Sometimes every week, so he builds miles quickly. If you don't fly a lot, it can take awhile to collect enough miles to do you any good.
For example, hubby just went somewhere and I think he got 2,500 points. It takes a minimum of 25,000 points to get one free ticket.
If you only fly once a year, you can tell that these miles would expire before you could accumulate enough to do any good.
American Airlines has their points expiring every 18 months if there is no "activity"...which means you have to fly at least once every 18 months to keep the points from disappearing.