Because of their small size ( 0.02 inches long), impressive reproductive capability (a termite queen products one egg every 3 seconds, or over 10 million eggs per year) https://blogs.scientificamerican.com/running-ponies/long-live-the-morbidly-obese-termite-queen-and-her-terrifying-army-of-sweat-licking-babies/, and ability to eat through many construction materials, termites are the most destructive of all insects.