I Can't speak for everyone but in my case, its not so much about getting more sp/xp from the same leve but rather having good leves when i do guild leve pts. To wait 36h for reset to get complete trash leves is a waste of my time, then i would have to wait another 36h for the chance of getting good leves. Its just easier for me to fail the good leves and do them again during reset.

Also you have to remember that most of these leves that people fail only give good sp/xp when it has at least 2 links; even if you fail the leve, then during reset u get the same leve again, sure you have the option to do it twice but that does not mean that everyone in your party also got the same leve again to do a 2nd run.