Okay, so why inches divided into 8ths?

And why are there 16 cups in a gallon, 15-and-some tablespoons in a cup and 3 teaspoons in a tablespoon?
Better make it 12 tablespoons in a cup and 12 cups in a gallon, then!

And why are there 14 pounds in a stone and 16 ounce in a pound?

The imperial system does not use dozenal.
It uses a clusterfuck of bases because it's actually a clusterfuck of measuring systems in a really big trenchcoat