A 2014 study found a correlation between striping and overlap with horse and tsetse fly populations and activity. Other studies have found that zebras are rarely targeted by these insect species. Caro and colleagues (2019) studied captive zebras and horses and observed that neither could deter flies from a distance, but zebra stripes kept flies from landing, both on zebras and horses dressed in zebra print coats. […] White or light stripes painted on dark bodies have also been found to reduce fly irritations in both cattle and humans.
Latest theory I heard was:
https://en.wikipedia.org/wiki/Zebra