The people who volunteer to be subject of those long-term studies tend to have little overlap with the demographic exposed to said chemicals. I.e., your average

The people who volunteer to be subject of those long-term studies tend to have little overlap with the demographic exposed to said chemicals. I.e., your average seasonal migrant who's going to work 8 years here then return to their South or Central American home. If they do stay here, they're more often than not seasonal migrants, sans documentation, not the type who'd volunteer to come into a university building twice a year for 20 years to be a medical subject of a study. (These guys likely don't even have base-level medical healthcare).