Small owls like this one can have eyes that take up almost half of the volume of the skull, and they don’t have nice dense bones or any padding most places because they traded all that for flight. That leaves birds, and owls in particular, very susceptible to injury.
This is why small birds can mob the predators without much fear of being attacked back. It is just too easy to get injured as a bird, and any needless fight can blind them, leave them unable to fly, or leave them too low on calories to have the energy to hunt their next meal.