Good questions, here are some more.
Why do fat chance and slim chance mean the same thing?
If you can't drink and drive, why do you need a driver's license to buy booze, and why do bars have parking lots?
Why do we drive on parkways and park on driveways?
Why is it called a TV "set" when you only get one?
Why does your nose run and your feet smell?
Why is it, whether you sit up or sit down, the result is the same?
Why call it a building if it's already been built?
How can there be self-help "groups"?
Isn't it a bit unnerving that doctors call what they do "practice"?
Why is a pear called a pear when there is only one?
Why do you wear a pair of panties and only one bra?
Why don't sheep shrink when it rains?
Why are they called apartments when they are all stuck together?
Why is it called a "near miss" when you don't hit something?
Why is abbreviated such a long word?
Why is there an 's' in lisp?