Good ones don’t! Most modern soft coolers have watertight zippers and seals, and openings that close with robust mechanisms. Most quality soft coolers also use materials and construction that avoid seams and junctures that could provide opportunities for leakage.  Still, the durability prize goes to hard coolers, so if you have zero tolerance for leakage and want to abuse your cooler and still get leak-proof performance, consider a hard cooler for your needs.