Couple thoughts
1) I had friends who thought it was too complicated the first time they went, so they didn't use it. The next time they went she asked me for some FP tips as they were traveling with family that included a child with mild disabilities. (They also used Disability services). They still didn't end up using FP a lot that trip because of the disability card, but they used it some. She thanked me for the tips.
2) People don't think about it, and don't realize how much time it will save. As someone mentioned they may just want to do all of Fantasyland at once. There are no FP in Fantasyland, so they don't get any for the first three hours while they hang out in Fantasyland and Toon Town, etc. Suddenly it's lunch time and by mid-afternoon the FP return times are for later in the day. They're thinking I don't want to go on Star Tours in 2 hours, I want to go now. They don't consider that going now means waiting an hour and that if they got the FP they could do a few other things in the two hours and then just wait 10 minutes.
3) People do use FP, but then they wait in line. We grab FP and then ride other rides or do other things that don't require excessive waits. But not everyone does that. Some may get a FP for Space Mountain that is ready in two hours and then go standby the Splash Mountain line that is two hours long. Why? Because they don't want to ride Buzz or Winnie the Pooh just because there isn't a line. I could imagine this would be especially the case with locals and others who go regularly. They don't have the need to do everything, or to do everything in the most efficient manner.
4) I also think people forget, they don't really think it saves that much time, they think waiting in line is part of the vacation, they think FP ruined
Disneyland and refuse out of principle (Ok maybe not). Who knows.
Overall most people I know use FP at least some of the time. It's just that most people aren't as efficient as those who have gone or go more regularly.