I think it's because when he wore different pants he believed he was no longer SpongeBob Squarepants so he has to wear the same pants everyday to avoid any confusion o at least an identity crisis. :))
Because his been using it for at the krusty krab,meeting mermaidman and barnacle boy,jellyfishing with patrick,karate with sandy,going to school,During Picture giorno and there someting different on his pants is to going to the goo lagoon,Being hall monitor,Dress up like squidward's grilfreind,Turn into a lumaca like gary,Patrick will be spongebob and spongebob will be patrick That is all and bye! bye!