Food poisoning is one good reason.
After a few hours in the sun, ordinary potato salad can develop bacteria the size of raccoons. Step one in planning your picnic is to decide on a menu. Martha Stewart has loads of innovative suggestions for unique, imaginative and tasty summer meals. So you can forget about her. Because you're having a traditional Fourth of July picnic, and that means a menu of hot dogs charred into cylinders of industrial-grade carbon, and hamburgers so undercooked that when people try to eat them, they leap off the plate and frolic on the lawn like otters.