Most folks don't HAVE to ask. They are just curious.
Why all the negativity every time some innocent soul asks the question?
In any case, it's an interesting and important question for many reasons. If I'm challenged to say what those reasons are, OK, for me they include budgeting, trip planning, calculating distance between needed stops, comparative engine performance, being aware of problems associated with poor mileage, and just plain ole intellectual curosity.
But every time the question is asked, the negativity will arise again as sure as iron filings to a magnet. Every time.