Latest theory I heard was:

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.

https://en.wikipedia.org/wiki/Zebra

The truth is we don't actually know because the zebras don't want us to:

So, the question why zebras have stripes have proven very difficult and not without risks – Stephen Cobb has been bitten in the arm and admitted to hospital twice. Despite the extra vigour of recent work, the answer remains inconclusive.