The problem is typically you ate a bunch of things in the time before becoming ill and don't know which one (or none) is the culprit. So it takes a government investigation to identify the cause by correlating many different victims' food consumption to find the common factor.

Even when it's totally obvious the cause can be hard to actually prove. A restaurant near me put several people in the hospital with a certain kind of mushroom a couple years back but investigations never conclusively proved the source.