Why are owl hoots low-pitched?

There was an owl hooting outside our house earlier, and it occurred to me that every other bird has a high-pitched call.

Ravens have a croak that could be considered low, but their loud call is a caw that's higher. I can't think of another bird with a call nearly as low as owls'.

Search engines are no help, mostly duplicates answering why they hoot. Why are owls' calls so much lower than other birds?