Why do sleeping pills wear off so quick ?

Depends on type: There are two types of insomnia; sleep induction and sleep maintenance. Many drugs like zolpidem are for sleep induction. They have a few short half life (2 hours) so often give 4 hours of sleep. There are other options like lunesta (eszopiclone) designed for sleep maintenance. Also depends on why you need them. If not done, get a sleep study and explore causes. Many things are best not treated with pills.