Flying insects have been found to carry a wide range of dangerous microorganisms all over their bodies, in their saliva and excreta, including E.coli, Salmonella, Klebsiella, Campylobacter and many others. Even a tiny dose of any of these is enough to cause serious disease. At the very least, insect contamination reduces product quality, making it unsellable.
By law, food that is sold must be wholesome and free from contamination. Companies are required to demonstrate due diligence, meaning that they have taken every precaution against contamination from flying insect pests.