TL;DR

The reason why hiding in plain sight is the best place to hide is because humans overlook and disregard the seemingly unimportant details.

My personal experience

When I would live with my family, I often became upset when they would eat my favorite snacks. Since the fridge is a tight space, it did not matter if I put my snacks at the very top of the shelf or at the very bottom; in all cases, they were eaten!

Then, out of reasoning, I concluded that if I placed the snacks in a food container that was something I knew my family did not life (often healthy food like vegetables, oatmeal, etc), then they would over look it.

AND IT WORKED!!!

And the best part is that I would leave the snacks right at eye-level, and they would not get eaten, all because the snacks were hidden under the veneer of canned beets.

Other stuff

Where does the hawk look? He looks in the barn, he looks in the attic, he looks in the cellar, he looks everywhere he would hide. But there are so many places it would never occur to a hawk to hide.