In the korok forest, why do the koroks run away from me when I get in their sight?

The Koroks that disappear before you can talk to them are just decoration to make it seem like there are more Koroks in the forest without having to program responses for all of them. It fits with in-game lore that Koroks are mystical, shy beings who do not talk to many outsiders. The ones that do talk to you tend to serve a purpose that would require them to interact with outsiders or travellers, like the shopkeeper or innkeeper.