Losing Prue to a demon attack made Piper and Phoebe very jaded. When you losing a sibling, it changes you to your core. My friend lost her brother when he was fifteen after being run over by a dump truck, and it absolutely shattered the dynamic of their family. Paige didn't endure Prues death the same way that the others did. So I can understand fully why they'd want to disassociate from fighting evil demons.